Consider hidden decls for isUsed checks.

This fixes pr17624.

A FIXME from Richard Smith:

It seems to me that the root cause is that a per-Decl 'used' flag doesn't
really make much sense in the way we use it now. I think we should either track
whether that particular declaration is used (with isUsed scanning the entire
redecl chain), or we should only have one flag for the entire redeclaration
chain (perhaps by always looking at the flag on either the most recent decl or
the canonical decl). Modeling it as "is this declaration or any previous
declaration used" is weird, and requires contortions like the loop at the end
of Sema::MarkFunctionReferenced.
Sema: Allow IndirectFieldDecl to appear in a non-type template argument

We would not identify pointer-to-member construction in a non-type
template argument if it was either a FieldDecl or a CXXMethodDecl.
However, this would incorrectly reject declarations that were injected
via an IndirectFieldDecl (e.g. a field inside of an anonymous union).

This fixes PR17657.