Fix some interactions between C++11 and C++14 features and using-declarations:

 * a dependent non-type using-declaration within a function template can be
   valid, as it can refer to an enumerator, so don't reject it in the template
   definition
 * we can partially substitute into a dependent using-declaration if it appears
   within a (local class in a) generic lambda within a function template, which
   means an UnresolvedUsing*Decl doesn't necessarily instantiate to a UsingDecl.

llvm-svn: 290071