[clang] [CIR][NFC] Simplify LexicalScope::cleanup() (PR #191034)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

The CI testing failed because one of the functions I removed was actually used until I removed it in https://github.com/llvm/llvm-project/pull/191029. I overlooked this dependency between the changes. I'll restart testing after the other change is merged.

https://github.com/llvm/llvm-project/pull/191034