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

[Joseph Huber via cfe-commits]

================
@@ -357,6 +357,7 @@ void Preprocessor::RegisterBuiltinMacros() {
   Ident__has_builtin = RegisterBuiltinMacro("__has_builtin");
   Ident__has_constexpr_builtin =
       RegisterBuiltinMacro("__has_constexpr_builtin");
+  Ident__has_target_builtin = RegisterBuiltinMacro("__has_target_builtin");
----------------
jhuber6 wrote:

I guess the usage would commonly be
```
#if defined(__has_target_builtin) && __has_target_builtin(foo)
```

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