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

[Aaron Ballman via cfe-commits]

================
@@ -96,6 +100,47 @@ the ``<cmath>`` header file to conditionally make a function constexpr whenever
 the constant evaluation of the corresponding builtin (for example,
 ``std::fmax`` calls ``__builtin_fmax``) is supported in Clang.
 
+``__has_target_builtin``
+------------------------
+
+This function-like macro takes a single identifier argument that is the name of
+a builtin function, a builtin pseudo-function (taking one or more type
+arguments), or a builtin template.
+It evaluates to 1 if the builtin is supported on the current target or 0 if not.
+
+``__has_builtin`` and ``__has_target_builtin`` behave identically for normal C++ compilations.
----------------
AaronBallman wrote:

> if the user's goal is to check that the builtin can be codegen'd on the current target being compiled, then yes they should use __has_target_builtin. if they want to confirm it can be parsed but not necessarily codegen'd, then they should use __has_builtin. if that's unclear let me know and i can try to improve the doc

Users don't typically think in terms of "parsed" and "codegenned", but more "works" and "doesn't work", which I think means "codegenned" in general. The line we're drawing here is pretty subtle, so perhaps more real world examples would help; it's hard to understand why a builtin can be parsed but cannot be used.

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