Add support for overloaded matchers. ie different matcher function signatures with the same name.

Summary:
Add support for overloaded matchers.
This composes with other features, like supporting polymorphic matchers.

Reviewers: klimek

CC: cfe-commits, revane

Differential Revision: http://llvm-reviews.chandlerc.com/D1188

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186836 91177308-0d34-0410-b5e6-96231b3b80d8