[clang] Upstream enum support (PR #136807)

[Ankur Ahir via cfe-commits]

Ankur-0429 wrote:

> I would also want to see tests that show uses of an enum, as well as fixed underlying type, scoped enums/etc.

Going to wait to confirm I did the "fixed underlying type" part first. 

If you have more specific enum use cases that we should test for, please let me know.

https://github.com/llvm/llvm-project/pull/136807