For a dependent variable template specialization, we don't build a
dependent Decl node or a DeclRefExpr to represent it. Instead, we
preserve the UnresolvedLookupExpr until instantiation.
However, this approach isn't ideal for constraint normalization. We
consider the qualifier during profiling, but since that's based on the
written code, it can introduce confusing differences, even when the
expressions resolve to the same declaration.
This change profiles the underlying VarTemplateDecl if
UnresolvedLookupExpr is used to model a dependent use of it.
Fixes https://github.com/llvm/llvm-project/issues/139476
2c2ad9a096