[clang] [libcxx] [Clang] Add __builtin_invoke and use it in libc++ (PR #116709)

[Aaron Ballman via cfe-commits]

================
@@ -4272,6 +4272,12 @@ def MoveIfNsoexcept : CxxLibBuiltin<"utility"> {
   let Namespace = "std";
 }
 
+def Invoke : Builtin {
+  let Spellings = ["__builtin_invoke"];
+  let Attributes = [CustomTypeChecking, Constexpr];
----------------
AaronBallman wrote:

We should still add test coverage to `clang/test/SemaCXX` for it though; we don't want to have to wait for us to break libc++ tests to know we've regressed anything here.

https://github.com/llvm/llvm-project/pull/116709