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

[Younan Zhang via cfe-commits]

zyn0217 wrote:

> 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.

Yeah, I added some explanation to the document of `UnresolvedLookupExpr`; further, I didn't see anything specific to the topic of `UnresolvedLookupExpr` in the `InternalsManual.rst`, so I hope this is sufficient?

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