[clang] [Clang] Add GCC's __builtin_stack_address() to Clang. (PR #121332)

[Aaron Ballman via cfe-commits]

================
@@ -899,6 +899,12 @@ def FrameAddress : Builtin {
   let Prototype = "void*(_Constant unsigned int)";
 }
 
+def StackAddress : Builtin {
+  let Spellings = ["__builtin_stack_address"];
+  let Attributes = [NoThrow];
----------------
AaronBallman wrote:

Should this be marked `Const` (or maybe `Pure`)?

https://github.com/llvm/llvm-project/pull/121332