[mlir][spirv] Add return to function to please MSVC

This suppresses the "not all control paths return a value" warning.

mlir/lib/Dialect/SPIRV/SPIRVLowering.cpp

@@ -18,6 +18,7 @@
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Debug.h"
 
+#include <cstdint>
 #include <functional>
 
 #define DEBUG_TYPE "mlir-spirv-lowering"
@@ -136,6 +137,8 @@ SPIRVTypeConverter::getMemorySpaceForStorageClass(spirv::StorageClass storage) {
 
   switch (storage) { STORAGE_SPACE_MAP_LIST(STORAGE_SPACE_MAP_FN) }
 #undef STORAGE_SPACE_MAP_FN
+  llvm_unreachable("unhandled storage class!");
+  return UINT32_MAX;
 }
 
 Optional<spirv::StorageClass>