[clang] [clang][amdgpu] mapCABIAtomicOrdering - fix MSVC not all control paths return a value warning. NFC. (PR #180500)

[Simon Pilgrim via cfe-commits]

================
@@ -290,6 +290,7 @@ static llvm::AtomicOrdering mapCABIAtomicOrdering(unsigned AO) {
   case llvm::AtomicOrderingCABI::relaxed:
     return llvm::AtomicOrdering::Monotonic;
   }
+  llvm_unreachable("Unknown AtomicOrderingCABI enum");
----------------
RKSimon wrote:

A default case would fix the MSVC warning, but IIRC gcc doesn't like switch(enum class) with an unnecessary default :/ 

https://github.com/llvm/llvm-project/pull/180500