Add TemplateSpecializationType polymorphism for hasTemplateArgument and
hasAnyTemplateArgument, and (out of necessity) an isExpr matcher.

Also updates the TemplateArgument doxygen to reflect reality for
non-canonical template arguments.

Differential Revision: http://llvm-reviews.chandlerc.com/D2810

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201804 91177308-0d34-0410-b5e6-96231b3b80d8