Traversing template paramter lists of DeclaratorDecls and/or TagDecls.

The unit tests in this patch demonstrate the need to traverse template
parameter lists of DeclaratorDecls (e.g. VarDecls, CXXMethodDecls) and
TagDecls (e.g. EnumDecls, RecordDecls).

Fixes PR29042.
https://reviews.llvm.org/D24268

Patch from Lukasz
Łukasz Anforowicz <lukasza@chromium.org>!

llvm-svn: 281345