Add a new flag, -fspell-checking-limit=<number> to control how many times we'll do spell checking. Note that spell checking will change the produced AST, so we don't automatically change this value when someone sets -ferror-limit=. With this, merge test typo-correction-pt2.cpp into typo-correction.cpp.

Remove Sema::UnqualifiedTyposCorrected, a cache of corrected typos. It would only cache typo corrections that didn't provide ValidateCandidate of which there were few left, and it had a bug when we had the same identifier spelled wrong twice. See the last two tests in typo-correction.cpp for cases this fires.