Implement descendant matchers for NestedNamespecifiers

This implements has(), hasDescendant(), forEach() and
forEachDescendant() for NestedNameSpecifier and NestedNameSpecifierLoc
matchers.

Review: http://llvm-reviews.chandlerc.com/D86

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@167017 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed