[CodeComplete] Expose InBaseClass signal in code completion results.

Summary:
No new tests as the existing tests for result priority should give us
coverage. Also as the new flag is trivial enough, I'm reluctant to plumb the
flag to c-index-test output.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D53635

llvm-svn: 345135