[Concepts] Fix an assertion failure while diagnosing constrained

function candidates

See: https://github.com/llvm/llvm-project/issues/54379

I tried to see if I can reuse ResolveAddressOfOverloadedFunction for
explicit function instantiation and so I managed to hit this ICE.

Bug was the diagnostic required an argument (%0) and specific code path
didn't pass an argument.

Differential Revision: https://reviews.llvm.org/D121646