[clang] [clang-tools-extra] [PAC][clang] Define `PointerAuthQualifier` and `PointerAuthenticationMode` (PR #84384)

[Daniil Kovalev via cfe-commits]

kovdan01 wrote:

I've fixed the build failure https://buildkite.com/llvm-project/github-pull-requests/builds/55986 in clang-tidy by only taking fast qualifiers from the `uint64_t` opaque value and casting the value to `unsigned` - see e3cb910ebce1ffa187816a20ce52e6303799665a. It should be OK to strip bits which not present in `FastMask` since `QualType` constructor seems to expect fast qualifiers only - see this in `QualType` definition:

```
llvm::PointerIntPair<llvm::PointerUnion<const Type *, const ExtQuals *>,
                     Qualifiers::FastWidth> Value;
// ...
QualType(const Type *Ptr, unsigned Quals) : Value(Ptr, Quals) {}
QualType(const ExtQuals *Ptr, unsigned Quals) : Value(Ptr, Quals) {}
```

@AaronBallman Please let me know if the approach from e3cb910ebce1ffa187816a20ce52e6303799665a is correct or if we should fix the issue somehow differently.

https://github.com/llvm/llvm-project/pull/84384