[clang] [Clang] Fixed UnresolvedLookupExpr propagating into the codegen phase (PR #124609)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I have minor suggestions, and am not quite comfortable with this yet, so please improve the commit message and make the change I requested (re `Resolvable`), and I'll revisit this afterwards.

https://github.com/llvm/llvm-project/pull/124609