[concepts] Preserve the FoundDecl of ConceptReference properly (#85032)

The `ConceptReference`'s `FoundDecl` claims it "can differ from
`NamedConcept` when, for example, the concept was found through a
`UsingShadowDecl`", but such the contract was not previously respected.

Fixes https://github.com/llvm/llvm-project/issues/82628