[llvm][DebugInfo][NFC] Abstract DICompileUnit::SourceLanguage to allow alternate DWARF SourceLanguage encoding (#162255)

This patch sets up `DICompileUnit` to support the DWARFv6
`DW_AT_language_name` and `DW_AT_language_version` attributes (which are
set to replace `DW_AT_language`). This patch changes the
`DICompileUnit::SourceLanguage` field type to a `DISourceLanguageName`
that encapsulates the notion of "versioned vs. unversioned name". A
"versioned" name is one that has an associated version stored separately
in `DISourceLanguageName::Version`.

This patch just changes all the clients of the `getSourceLanguage` API
to the expect a `DISourceLanguageName`. Currently they all just `assert`
(via `DISourceLanguageName::getUnversionedName`) that we're dealing with
"unversioned names" (i.e., the pre-DWARFv6 language codes). In follow-up
patches (e.g., draft is at
https://github.com/llvm/llvm-project/pull/162261), when we start
emitting versioned language codes, the `getUnversionedName` calls can
then be adjusted to `getName`.

**Implementation considerations**

* We could have added a new member to `DICompileUnit` alongside the
existing `SourceLanguage` field. I don't think this would have made the
transition any simpler (clients would still need to be aware of
"versioned" vs. "unversioned" language names). I felt that encapsulating
this inside a `DISourceLanguageName` was easier to reason about for
maintainers.
* Currently DISourceLanguageName is a `12` byte structure. We could
probably pack all the info inside a `uint64_t` (16-bits for the name,
32-bits for the version, 1-bit for answering the `hasVersionedName`).
Just to keep the prototype simple I used a `std::optional`. But since
the guts of the structure are hidden, we can always change the layout to
a more compact representation instead.

**How to review**

* The new `DISourceLanguageName` structure is defined in
`DebugInfoMetadata.h`. All the other changes fall out from changing the
`DICompileUnit::SourceLanguage` from `unsigned` to
`DISourceLanguageName`.

1 files changed, 5 insertions, 4 deletions

diff --git a/llvm/lib/IR/DebugInfoMetadata.cpp b/llvm/lib/IR/DebugInfoMetadata.cpp
index 77d044b..e30df88 100644
--- a/llvm/lib/IR/DebugInfoMetadata.cpp
+++ b/llvm/lib/IR/DebugInfoMetadata.cpp
@@ -1184,9 +1184,10 @@ DIFile *DIFile::getImpl(LLVMContext &Context, MDString *Filename,
   DEFINE_GETIMPL_STORE(DIFile, (CS, Source), Ops);
 }
 DICompileUnit::DICompileUnit(LLVMContext &C, StorageType Storage,
-                             unsigned SourceLanguage, bool IsOptimized,
-                             unsigned RuntimeVersion, unsigned EmissionKind,
-                             uint64_t DWOId, bool SplitDebugInlining,
+                             DISourceLanguageName SourceLanguage,
+                             bool IsOptimized, unsigned RuntimeVersion,
+                             unsigned EmissionKind, uint64_t DWOId,
+                             bool SplitDebugInlining,
                              bool DebugInfoForProfiling, unsigned NameTableKind,
                              bool RangesBaseAddress, ArrayRef<Metadata *> Ops)
     : DIScope(C, DICompileUnitKind, Storage, dwarf::DW_TAG_compile_unit, Ops),
@@ -1199,7 +1200,7 @@ DICompileUnit::DICompileUnit(LLVMContext &C, StorageType Storage,
 }
 
 DICompileUnit *DICompileUnit::getImpl(
-    LLVMContext &Context, unsigned SourceLanguage, Metadata *File,
+    LLVMContext &Context, DISourceLanguageName SourceLanguage, Metadata *File,
     MDString *Producer, bool IsOptimized, MDString *Flags,
     unsigned RuntimeVersion, MDString *SplitDebugFilename,
     unsigned EmissionKind, Metadata *EnumTypes, Metadata *RetainedTypes,