[clang] [Clang] Add __has_target_builtin macro (PR #126324)

[Nick Sarnie via cfe-commits]

================
@@ -1819,8 +1822,12 @@ void Preprocessor::ExpandBuiltinMacro(Token &Tok) {
               // usual allocation and deallocation functions. Required by libc++
               return 201802;
             default:
+              // __has_target_builtin should return false for aux builtins.
+              if (IsHasTargetBuiltin &&
+                  getBuiltinInfo().isAuxBuiltinID(BuiltinID))
----------------
sarnex wrote:

Good question. The function name is `isAuxBuiltinID` but the implementation of it makes it so that the behavior is like `isOnlyAuxBuiltinID`. The implementation of the function is:
```
 /// Return true if builtin ID belongs to AuxTarget.
 bool isAuxBuiltinID(unsigned ID) const {  
   return ID >= (Builtin::FirstTSBuiltin + NumTargetBuiltins); 
  } 
```

When we register builtins, we do it like this:

```
TargetShards = Target.getTargetBuiltins();
  for (const auto &Shard : TargetShards)
    NumTargetBuiltins += Shard.Infos.size();
  if (AuxTarget) {
    AuxTargetShards = AuxTarget->getTargetBuiltins();
    for (const auto &Shard : AuxTargetShards)
      NumAuxTargetBuiltins += Shard.Infos.size();
  }
}
```

So we register all the target builtins before the aux target builtins, and we only consider something an aux builtin if it was registered specifically as an aux builtin, so I think the logic does what we want.

https://github.com/llvm/llvm-project/pull/126324