[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, 5 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index c6d9fcf..316e05ba 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -1043,6 +1043,11 @@ void DwarfUnit::constructTypeDIE(DIE &Buffer, const DICompositeType *CTy) {
         addUInt(Buffer, dwarf::DW_AT_calling_convention, dwarf::DW_FORM_data1,
                 CC);
     }
+
+    if (auto *SpecifiedFrom = CTy->getSpecification())
+      addDIEEntry(Buffer, dwarf::DW_AT_specification,
+                  *getOrCreateContextDIE(SpecifiedFrom));
+
     break;
   }
   default: