[DebugInfo] Add a specification attribute to LLVM DebugInfo (#115362)

Add a specification attribute to LLVM DebugInfo, which is analogous
to DWARF's DW_AT_specification. According to the DWARF spec:
"A debugging information entry that represents a declaration that
completes another (earlier) non-defining declaration may have a
DW_AT_specification attribute whose value is a reference to the
debugging information entry representing the non-defining declaration."

This patch allows types to be specifications of other types. This is
used by Swift to represent generic types. For example, given this Swift
program:

```
struct MyStruct<T> {
    let t: T
}

let variable = MyStruct<Int>(t: 43)
```

The Swift compiler emits (roughly) an unsubtituted type for MyStruct<T>:
```
DW_TAG_structure_type
    DW_AT_name	("MyStruct")
    // "$s1w8MyStructVyxGD" is a Swift mangled name roughly equivalent to 
    // MyStruct<T>
    DW_AT_linkage_name	("$s1w8MyStructVyxGD")
    // other attributes here
```
And a specification for MyStruct<Int>:
```
DW_TAG_structure_type
    DW_AT_specification	(<link to "MyStruct">)
    // "$s1w8MyStructVySiGD" is a Swift mangled name equivalent to
    // MyStruct<Int>
    DW_AT_linkage_name	("$s1w8MyStructVySiGD")
    DW_AT_byte_size	(0x08)
    // other attributes here
```

1 files changed, 22 insertions, 21 deletions

diff --git a/llvm/lib/IR/DebugInfoMetadata.cpp b/llvm/lib/IR/DebugInfoMetadata.cpp
index f7268a5..915cdd3 100644
--- a/llvm/lib/IR/DebugInfoMetadata.cpp
+++ b/llvm/lib/IR/DebugInfoMetadata.cpp
@@ -770,21 +770,21 @@ DICompositeType *DICompositeType::getImpl(
     Metadata *Elements, unsigned RuntimeLang, Metadata *VTableHolder,
     Metadata *TemplateParams, MDString *Identifier, Metadata *Discriminator,
     Metadata *DataLocation, Metadata *Associated, Metadata *Allocated,
-    Metadata *Rank, Metadata *Annotations, uint32_t NumExtraInhabitants,
-    StorageType Storage, bool ShouldCreate) {
+    Metadata *Rank, Metadata *Annotations, Metadata *Specification,
+    uint32_t NumExtraInhabitants, StorageType Storage, bool ShouldCreate) {
   assert(isCanonical(Name) && "Expected canonical MDString");
 
   // Keep this in sync with buildODRType.
-  DEFINE_GETIMPL_LOOKUP(DICompositeType,
-                        (Tag, Name, File, Line, Scope, BaseType, SizeInBits,
-                         AlignInBits, OffsetInBits, Flags, Elements,
-                         RuntimeLang, VTableHolder, TemplateParams, Identifier,
-                         Discriminator, DataLocation, Associated, Allocated,
-                         Rank, Annotations, NumExtraInhabitants));
+  DEFINE_GETIMPL_LOOKUP(
+      DICompositeType,
+      (Tag, Name, File, Line, Scope, BaseType, SizeInBits, AlignInBits,
+       OffsetInBits, Flags, Elements, RuntimeLang, VTableHolder, TemplateParams,
+       Identifier, Discriminator, DataLocation, Associated, Allocated, Rank,
+       Annotations, Specification, NumExtraInhabitants));
   Metadata *Ops[] = {File,          Scope,        Name,           BaseType,
                      Elements,      VTableHolder, TemplateParams, Identifier,
                      Discriminator, DataLocation, Associated,     Allocated,
-                     Rank,          Annotations};
+                     Rank,          Annotations,  Specification};
   DEFINE_GETIMPL_STORE(DICompositeType,
                        (Tag, Line, RuntimeLang, SizeInBits, AlignInBits,
                         OffsetInBits, NumExtraInhabitants, Flags),
@@ -795,10 +795,11 @@ DICompositeType *DICompositeType::buildODRType(
     LLVMContext &Context, MDString &Identifier, unsigned Tag, MDString *Name,
     Metadata *File, unsigned Line, Metadata *Scope, Metadata *BaseType,
     uint64_t SizeInBits, uint32_t AlignInBits, uint64_t OffsetInBits,
-    uint32_t NumExtraInhabitants, DIFlags Flags, Metadata *Elements,
-    unsigned RuntimeLang, Metadata *VTableHolder, Metadata *TemplateParams,
-    Metadata *Discriminator, Metadata *DataLocation, Metadata *Associated,
-    Metadata *Allocated, Metadata *Rank, Metadata *Annotations) {
+    Metadata *Specification, uint32_t NumExtraInhabitants, DIFlags Flags,
+    Metadata *Elements, unsigned RuntimeLang, Metadata *VTableHolder,
+    Metadata *TemplateParams, Metadata *Discriminator, Metadata *DataLocation,
+    Metadata *Associated, Metadata *Allocated, Metadata *Rank,
+    Metadata *Annotations) {
   assert(!Identifier.getString().empty() && "Expected valid identifier");
   if (!Context.isODRUniquingDebugTypes())
     return nullptr;
@@ -809,8 +810,7 @@ DICompositeType *DICompositeType::buildODRType(
                AlignInBits, OffsetInBits, Flags, Elements, RuntimeLang,
                VTableHolder, TemplateParams, &Identifier, Discriminator,
                DataLocation, Associated, Allocated, Rank, Annotations,
-               NumExtraInhabitants);
-
+               Specification, NumExtraInhabitants);
   if (CT->getTag() != Tag)
     return nullptr;
 
@@ -825,7 +825,7 @@ DICompositeType *DICompositeType::buildODRType(
   Metadata *Ops[] = {File,          Scope,        Name,           BaseType,
                      Elements,      VTableHolder, TemplateParams, &Identifier,
                      Discriminator, DataLocation, Associated,     Allocated,
-                     Rank,          Annotations};
+                     Rank,          Annotations,  Specification};
   assert((std::end(Ops) - std::begin(Ops)) == (int)CT->getNumOperands() &&
          "Mismatched number of operands");
   for (unsigned I = 0, E = CT->getNumOperands(); I != E; ++I)
@@ -838,10 +838,11 @@ DICompositeType *DICompositeType::getODRType(
     LLVMContext &Context, MDString &Identifier, unsigned Tag, MDString *Name,
     Metadata *File, unsigned Line, Metadata *Scope, Metadata *BaseType,
     uint64_t SizeInBits, uint32_t AlignInBits, uint64_t OffsetInBits,
-    uint32_t NumExtraInhabitants, DIFlags Flags, Metadata *Elements,
-    unsigned RuntimeLang, Metadata *VTableHolder, Metadata *TemplateParams,
-    Metadata *Discriminator, Metadata *DataLocation, Metadata *Associated,
-    Metadata *Allocated, Metadata *Rank, Metadata *Annotations) {
+    Metadata *Specification, uint32_t NumExtraInhabitants, DIFlags Flags,
+    Metadata *Elements, unsigned RuntimeLang, Metadata *VTableHolder,
+    Metadata *TemplateParams, Metadata *Discriminator, Metadata *DataLocation,
+    Metadata *Associated, Metadata *Allocated, Metadata *Rank,
+    Metadata *Annotations) {
   assert(!Identifier.getString().empty() && "Expected valid identifier");
   if (!Context.isODRUniquingDebugTypes())
     return nullptr;
@@ -851,7 +852,7 @@ DICompositeType *DICompositeType::getODRType(
         Context, Tag, Name, File, Line, Scope, BaseType, SizeInBits,
         AlignInBits, OffsetInBits, Flags, Elements, RuntimeLang, VTableHolder,
         TemplateParams, &Identifier, Discriminator, DataLocation, Associated,
-        Allocated, Rank, Annotations, NumExtraInhabitants);
+        Allocated, Rank, Annotations, Specification, NumExtraInhabitants);
   } else {
     if (CT->getTag() != Tag)
       return nullptr;