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.

llvm-svn: 193203