Give the default CorrectionCandidateCallback::ValidateCandidate some
smarts so that it doesn't approve of keywords and/or type names when it
knows (based on its flags) that those kinds of corrections are not
wanted.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178668 91177308-0d34-0410-b5e6-96231b3b80d8