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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > However, will this actually work in practice in the debugger? If not, perhaps we should limit to just integer and enumeration types for now, leaving the extension for the future.
> 
> I composed an example of that:
> 
> ```c++
> struct A {
>   short a1;
>   short a2;
> };
> 
> struct B {
>   [[clang::preferred_type(A)]] unsigned b1 : 32 = 0x000F'000C;
> };
> 
> int main()
> {
>     B b;
>     return b.b1;
> }
> ```
> 
> Nightly build of LLDB from apt.llvm.org handles it just fine:
> 
> ```
> Process 2755547 stopped
> * thread #1, name = 'test-preferred-', stop reason = step in
>     frame #0: 0x0000555555555148 test-preferred-type`main at test.cxx:13:14
>    10   int main()
>    11   {
>    12       B b;
> -> 13       return b.b1;
>    14   }
> (lldb) v -T
> (B) b = {
>   (A:32) b1 = {
>     (short) a1 = 12
>     (short) a2 = 15
>   }
> }
> ```
> 
> @erichkeane Would you still prefer restricting type argument to integral and enumeration types?

I think that is the least dangerous, but the `requireCompleteType` isn't awful (and I can do a disagree & commit on).  I think it gets goofy when the sizes don't match with floating point/complex/etc, but I also see "you got what you asked for!" as being a reasonable defense to that.

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