[ASTMatchers] isTemplateInstantiation: also match explicit instantiation declaration.

Summary:
Example:
template <typename T> class X {}; class A {};
// Explicit instantiation declaration.
extern template class X<A>;

Reviewers: bkramer

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D43567

llvm-svn: 325678