[clang] Revert "[Clang] Deleting an incomplete enum type is not an error (#118455) (PR #118980)

[Alexander Kornienko via cfe-commits]

alexfh wrote:

I've added a reduced test case to https://github.com/llvm/llvm-project/pull/118455. Merging this to unbreak clang.

https://github.com/llvm/llvm-project/pull/118980