[clang] Upstream enum support (PR #136807)

[Erich Keane via cfe-commits]

erichkeane 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.

You have not.  Something like:

`enum E : int;`
`enum F : int {...};`
`enum class G : int;`
/etc

Note the first two are valid in C, and I'd like to make sure we get them in various stages of 'completeness' during emitting.

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