[clang] [llvm] [AMDGPU][clang][CodeGen][opt] Add late-resolved feature identifying predicates (PR #134016)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

First, please take a look at the LLVM coding standard re the use of 'auto'.

Second: The use of a special type for these builtins is a little novel (though I see the predicate type already exists?), but I guess I'm ok with it.  I have some concerns with how the conversions for it work, particularly being represented always as an `i1`, but the tests you have look about right.

I would like to see a test that is effectively:

```
bool f() {
return __builtin_amdgcn_processor_is(...);
}
```

(and maybe one returning 'auto' to make sure it is deduced properly).

https://github.com/llvm/llvm-project/pull/134016