[Clang][Sema] Skip checking anonymous enum in using enum declaration (#87144)

Try to fix https://github.com/llvm/llvm-project/issues/86790


`getFETokenInfo` requires `DeclarationName` shouldn't be empty and this
will produce crash when checking name conflict of an anonymous
`NamedDecl` in `Sema::PushOnScopeChains` and whether it's a reserved
identifier or not. These wouldn't happen when it's a anonymous enum and
we can skip the checking and just add the declaration to current scope.

Co-authored-by: huqizhi <836744285@qq.com>