Fix a typo correction crash when resolving ambiguous corrections.

In certain cases, the tree transform would introduce new TypoExprs while
trying one of the corrections, invalidating the unique_ptr in the state
reference, and also causing a TypoExpr to exist that will never be
corrected since it doesn't exist in the final corrected expression. The
simple solution to both problems is to temporarily disable typo
correction while handling potentially ambiguous typo corrections.

llvm-svn: 240734