Add "equalsNode" for types and "isCopyAssignmentOperator" matchers.

Summary: This matchers are going to be used in modernize-use-default, but are generic enough to be placed in ASTMatchers.h.

Reviewers: klimek

Subscribers: alexfh, cfe-commits, klimek

Differential Revision: http://reviews.llvm.org/D14152

llvm-svn: 251693