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

[Aaryaman Sagar via cfe-commits]

aary wrote:

Hi @jeremy-rifkin ! Are you still pushing on this PR? I work at Meta, and lack of warnings in this case caused a fairly scary outage for us earlier this week :( 

I was discussing this issue with other folks internally, and they pointed me to this PR, which would solve our problem very precisely! So we got excited to see where this goes. 

Thanks! 

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