[mlir] Properly align StorageUniquer::BaseStorage to fix 32 bit build

We allow stealing up to 3 bits of pointers to BaseStorage, so we need to make sure that we align by at least 8.

mlir/include/mlir/Support/StorageUniquer.h

@@ -81,7 +81,7 @@ class StorageUniquer {
 public:
   /// This class acts as the base storage that all storage classes must derived
   /// from.
-  class BaseStorage {
+  class alignas(8) BaseStorage {
   protected:
     BaseStorage() = default;
   };