[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, 11 insertions, 5 deletions

diff --git a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
index 8ca46a4..1caa848 100644
--- a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
+++ b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
@@ -1600,7 +1600,7 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     break;
   }
   case bitc::METADATA_COMPOSITE_TYPE: {
-    if (Record.size() < 16 || Record.size() > 23)
+    if (Record.size() < 16 || Record.size() > 24)
       return error("Invalid record");
 
     // If we have a UUID and this is not a forward declaration, lookup the
@@ -1630,6 +1630,7 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     Metadata *Allocated = nullptr;
     Metadata *Rank = nullptr;
     Metadata *Annotations = nullptr;
+    Metadata *Specification = nullptr;
     auto *Identifier = getMDString(Record[15]);
     // If this module is being parsed so that it can be ThinLTO imported
     // into another module, composite types only need to be imported as
@@ -1678,14 +1679,18 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
       if (Record.size() > 21) {
         Annotations = getMDOrNull(Record[21]);
       }
+      if (Record.size() > 23) {
+        Specification = getMDOrNull(Record[23]);
+      }
     }
     DICompositeType *CT = nullptr;
     if (Identifier)
       CT = DICompositeType::buildODRType(
           Context, *Identifier, Tag, Name, File, Line, Scope, BaseType,
-          SizeInBits, AlignInBits, OffsetInBits, NumExtraInhabitants, Flags,
-          Elements, RuntimeLang, VTableHolder, TemplateParams, Discriminator,
-          DataLocation, Associated, Allocated, Rank, Annotations);
+          SizeInBits, AlignInBits, OffsetInBits, Specification,
+          NumExtraInhabitants, Flags, Elements, RuntimeLang, VTableHolder,
+          TemplateParams, Discriminator, DataLocation, Associated, Allocated,
+          Rank, Annotations);
 
     // Create a node if we didn't get a lazy ODR type.
     if (!CT)
@@ -1694,7 +1699,8 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
                             SizeInBits, AlignInBits, OffsetInBits, Flags,
                             Elements, RuntimeLang, VTableHolder, TemplateParams,
                             Identifier, Discriminator, DataLocation, Associated,
-                            Allocated, Rank, Annotations, NumExtraInhabitants));
+                            Allocated, Rank, Annotations, Specification,
+                            NumExtraInhabitants));
     if (!IsNotUsedInTypeRef && Identifier)
       MetadataList.addTypeRef(*Identifier, *cast<DICompositeType>(CT));