Silence a not all control paths return a value diagnostic; NFC

Author: AaronBallman

clang/include/clang/Basic/Attr.td

@@ -4500,6 +4500,7 @@ def HLSLShader : InheritableAttr {
       case HLSLShaderAttr::Mesh:          return llvm::Triple::Mesh;
       case HLSLShaderAttr::Amplification: return llvm::Triple::Amplification;
     }
+    llvm_unreachable("unknown enumeration value");
   }
 }];
 }