[clang] [clang] Add clang::debug_info_type attribute for bitfields (PR #69104)

[via cfe-commits]

philnik777 wrote:

> > Wouldn't it be better to go the other way around? i.e. have a `[[clang::compressed_bitfield]]` (or whatever) which influences the ABI, so it's possible to do stuff like
> > ```
> > [[clang::compressed_bitfield]] bool IsSomething : 1;
> > [[clang::compressed_bitfield]] MyEnum Whatever : 3;
> > [[clang::compressed_bitfield]] int MoreStuff : 4;
> > ```
> > 
> > 
> > which the current approach doesn't allow.
> 
> The issue is that MSVC wouldn't know/recognize/implement this attribute - so clang couldn't do that without breaking ABI compatibility with MSVC.

That's not important for everybody, and you could still `#if` around that code like
```c++
#if __has_cpp_attribute(clang::compressed_bitfield)
#define BITFILED_T(ActualType, ABIType) static_assert(check_abi_compat<ActualType, ABIType>()); [[clang::compressed_bitfield]] ActualType
#else
#define BITFIELD_T(ActualType, ABIType) ABIType
...
```
Sure, it's not super pretty, but it gets you there and allows you to do more stuff. We could also ask the MSVC folks whether there would be interest to add such an attribute, since there is clearly interest in having a good ABI and stronger typing (CC @StephanTLavavej @CaseyCarter).

https://github.com/llvm/llvm-project/pull/69104