[llvm] [Arm64EC] Add support for `half` (PR #152843)

[Trevor Gross via llvm-commits]

================
@@ -316,6 +316,11 @@ ThunkArgInfo AArch64Arm64ECCallLowering::canonicalizeThunkType(
                         ThunkArgTranslation::PointerIndirection};
   };
 
+  if (T->isHalfTy()) {
+    Out << "h";
----------------
tgross35 wrote:

I've changed these to `__llvm_h__` and `__llvm_H__` for now, this is easy enough to update later if needed.

https://github.com/llvm/llvm-project/pull/152843