RecursiveASTVisitor: Visit instantiations of member templates of class

templates. In an implicit instantiation of a member class, any member
templates don't get instantiated, so the existing check which only visited
the instantiations of a defined template skipped these templates'
instantiations.

Since there is only a single declaration of a member template of a class
template specialization, just use that to determine whether to visit the
instantiations. This introduces a slight inconsistency in that we will
visit the instantiations of such templates whether or not they are
defined, but we never visit a declared-but-not-defined instantiation, so
this turns out to not matter.

Patch by Daniel Jasper!

llvm-svn: 155487