[ASTMatchers] Add isAssignmentOperator matcher

Adding a matcher for BinaryOperator and cxxOperatorCallExpr to be able to
decide whether it is any kind of assignment operator or not. This would be
useful since allows us to easily detect assignments via matchers for static
analysis (Tidy, SA) purposes.

Differential Revision: https://reviews.llvm.org/D44893

llvm-svn: 328618