[clang] fix obtaining EnumDecl for UsingEnumDecl (#156127)
Use the castAs acessor for the type for a UsingEnumDecl, as it can be
sugar for an EnumType.
Fixes a regression reported here:
https://github.com/llvm/llvm-project/pull/155313#issuecomment-3238482327
Since this regression was never released, there are no release notes.
