[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, 2 insertions, 6 deletions

diff --git a/llvm/lib/IR/DIBuilder.cpp b/llvm/lib/IR/DIBuilder.cpp
index 1344df9..1ae20a9f 100644
--- a/llvm/lib/IR/DIBuilder.cpp
+++ b/llvm/lib/IR/DIBuilder.cpp
@@ -131,17 +131,13 @@ static DIScope *getNonCompileUnitScope(DIScope *N) {
 }
 
 DICompileUnit *DIBuilder::createCompileUnit(
-    unsigned Lang, DIFile *File, StringRef Producer, bool isOptimized,
-    StringRef Flags, unsigned RunTimeVer, StringRef SplitName,
+    DISourceLanguageName Lang, DIFile *File, StringRef Producer,
+    bool isOptimized, StringRef Flags, unsigned RunTimeVer, StringRef SplitName,
     DICompileUnit::DebugEmissionKind Kind, uint64_t DWOId,
     bool SplitDebugInlining, bool DebugInfoForProfiling,
     DICompileUnit::DebugNameTableKind NameTableKind, bool RangesBaseAddress,
     StringRef SysRoot, StringRef SDK) {
 
-  assert(((Lang <= dwarf::DW_LANG_Metal && Lang >= dwarf::DW_LANG_C89) ||
-          (Lang <= dwarf::DW_LANG_hi_user && Lang >= dwarf::DW_LANG_lo_user)) &&
-         "Invalid Language tag");
-
   assert(!CUNode && "Can only make one compile unit per DIBuilder instance");
   CUNode = DICompileUnit::getDistinct(
       VMContext, Lang, File, Producer, isOptimized, Flags, RunTimeVer,