Qualify all types used in AST matcher macros.

Summary:
Qualify all types used in AST matcher macros. This makes it possible to
put AST matchers in user code into a namespace other than clang::ast_matchers
and this way prevent ODR violations that could happen when a matcher with the
same name is defined in multiple translation units. Updated comments
accordingly.

Reviewers: djasper, klimek

Reviewed By: djasper, klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D10501

llvm-svn: 239902