[clang] [CIR] Upstream a basic version of class LexicalScope (PR #131945)

[David Olsen via cfe-commits]

dkolsen-pgi wrote:

There are some small functions in this change that are not called or otherwise used in this change.  But they will definitely be used in future upstream patches.  Since they are small and don't bring in any new dependencies and are complete (won't need to be changed in future upstream patches), it seemed prudent to include them in this change.


https://github.com/llvm/llvm-project/pull/131945