Refactor "MatcherList" into "VariantMatcher" and abstract the notion of a list of matchers for the polymorphic case.

Summary:
Refactor "MatcherList" into "VariantMatcher" and abstract the notion of a list of matchers for the polymorphic case.
This work is to support future changes needed for eachOf/allOf/anyOf matchers. We will add a new type on VariantMatcher.

Reviewers: klimek

CC: cfe-commits, revane

Differential Revision: http://llvm-reviews.chandlerc.com/D1365

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188272 91177308-0d34-0410-b5e6-96231b3b80d8