[clang] Don't treat the default switch path as unreachable when all enumerators are covered (PR #123166)

[Erich Keane via cfe-commits]

erichkeane wrote:

Hmm... This isn't really my area of expertise, but I consider those types of warnings valuable. While it isn't technically 'unreachable', it IS by intent of the code.  Is there perhaps some sort of annotation on the struct we have that we could skip this assumption on instead?  Something that says "this enum might have values not in the enumerator list"?

https://github.com/llvm/llvm-project/pull/123166