Fix completion logic to allow for heterogeneous argument types in matcher overloads.

Summary:
There was an assumption that there were no matchers that were overloaded
on matchers and other types of arguments.
This assumption was broken recently with the addition of new matcher
overloads.

Fixes http://llvm.org/PR21226

Reviewers: pcc

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D5711