Fix the caching in CorrectTypo so that other non-keyword identifiers

are still added if the cached correction fails validation.

Also fix a copy-and-paste error in a comment from my previous commit.
Finally, add an example of the benefit the typo correction callback adds
to TryNamespaceTypoCorrection--which happens to also tickle the above
caching problem, as the only way a non-namespace Decl would be added to
the possible corrections is if it was cached as the correction for a
previous instance of the same typo where the typo was corrected to a
non-namespace via a different code path.

llvm-svn: 147968