RecursiveASTVisitor: When in 'shouldVisitTemplateInstantiations' mode, visit

all instantiations of a template when we visit the canonical declaration of the
primary template, rather than trying to match them up to the partial
specialization from which they are instantiated. This fixes a bug where we
failed to visit instantiations of partial specializations of member templates of
class templates, and naturally extends to allow us to visit instantiations where
we have instantiated only a declaration.

llvm-svn: 155597