[clang] [llvm] [Clang] Allow vector and matrix type attributes for sub-byte _BitInt (PR #140253)

[Dmitry Sidorov via cfe-commits]

MrSidims wrote:

> "tests would be a PITA to write, so lets restrict it and let someone who cares loosen it"

Yeah, that is understandable, I pushed the PR without extended tests just to check, if there won't be immediate objections before committing time to write them.

Soo... I have extended clang/test/CodeGenCXX/ext-int.cpp with quite obvious copy-paste like test `VectorTest` and may be not needed yet interesting for me case - check what clang would generate for stack allocations `TestBitIntVector2x2Alloca` (added few inline comments in the tests as well). Also few tests in clang/test/CodeGenCXX/matrix-vector-bit-int.cpp and a small test in X86 CodeGen. In close future we will also implement 4-bit integer type in SPIR-V backend, so extra tests will be there.

@erichkeane please advise if anything is missing. 

Ah, while I was writing - you have already approved :D

https://github.com/llvm/llvm-project/pull/140253