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

[Thibault Monnier via cfe-commits]

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

It seems to me there is only a warning when enabling `-Wswitch-enum`, but then, that's basically the point of the flag.

What gcc _does_ do is that it never considers a `switch case` without a `default` as handling all control paths, see https://godbolt.org/z/E8PcGoPv6. However, that's not applicable in this case. 

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