[clang] [clang] Accept empty enum in MSVC compatible C (PR #159981)

[Erich Keane via cfe-commits]

https://github.com/erichkeane requested changes to this pull request.

IF we're accepting this as an extension, we need to emit a diagnostic to that effect.  See the Ext-Warn already with an underlying type (which we already supported), we need to do something similar for these.

https://github.com/llvm/llvm-project/pull/159981