[llvm] [AArch64] Fixes for Windows varargs handling. (PR #139972)

[Reid Kleckner via llvm-commits]

================
@@ -9439,7 +9439,8 @@ AArch64TargetLowering::LowerCall(CallLoweringInfo &CLI,
     }
   }
 
-  if (IsVarArg && Subtarget->isWindowsArm64EC()) {
+  if (IsVarArg && Subtarget->isWindowsArm64EC() &&
+      !(CLI.CB && CLI.CB->isMustTailCall())) {
----------------
rnk wrote:

I see you have the test case to show that we forward X4 and X5 through musttail. 👍 

https://github.com/llvm/llvm-project/pull/139972