[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, 6 insertions, 4 deletions

diff --git a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
index 22c7fa5..a4d1b83 100644
--- a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
+++ b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
@@ -1866,11 +1866,13 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     // Ignore Record[0], which indicates whether this compile unit is
     // distinct.  It's always distinct.
     IsDistinct = true;
+
     auto *CU = DICompileUnit::getDistinct(
-        Context, Record[1], getMDOrNull(Record[2]), getMDString(Record[3]),
-        Record[4], getMDString(Record[5]), Record[6], getMDString(Record[7]),
-        Record[8], getMDOrNull(Record[9]), getMDOrNull(Record[10]),
-        getMDOrNull(Record[12]), getMDOrNull(Record[13]),
+        Context, DISourceLanguageName(Record[1]), getMDOrNull(Record[2]),
+        getMDString(Record[3]), Record[4], getMDString(Record[5]), Record[6],
+        getMDString(Record[7]), Record[8], getMDOrNull(Record[9]),
+        getMDOrNull(Record[10]), getMDOrNull(Record[12]),
+        getMDOrNull(Record[13]),
         Record.size() <= 15 ? nullptr : getMDOrNull(Record[15]),
         Record.size() <= 14 ? 0 : Record[14],
         Record.size() <= 16 ? true : Record[16],