[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@325678 91177308-0d34-0410-b5e6-96231b3b80d8