Add Distance parameter to ASTNodeKind::isBaseOf.

This will allow the completer to order results by relevance.

Differential Revision: http://llvm-reviews.chandlerc.com/D2209

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195540 91177308-0d34-0410-b5e6-96231b3b80d8