[clang] [clang] Distinguish unresolved templates in UnresolvedLookupExpr (PR #89019)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

This is introducing a new type that has a pretty subtle difference vs others.  Can we have some documentation in the internals manual explaining the difference?

Code wise, I think this looks fine.

https://github.com/llvm/llvm-project/pull/89019