[clang] [Clang][SPIRV] Add __spirv_event_t builtin type (PR #207077)

[Tom Honermann via cfe-commits]

https://github.com/tahonermann commented:

This is looking great. Tests look good. Perhaps a test for debuginfo should be added? I wouldn't know how to write one, but Brock would.

I think these files should have updates as well. I see uses of `EXT_OPAQUE_TYPE` in them too.
- `clang/lib/AST/MicrosoftMangle.cpp`
- `clang/lib/CIR/CodeGen/CIRGenItaniumCXXABI.cpp`
- `clang/lib/CodeGen/QualTypeMapper.cpp`
- `clang/tools/libclang/CXType.cpp`

https://github.com/llvm/llvm-project/pull/207077