[Sema][Typo Correction] Fix potential infite loop on ambiguity checks
This fixes a bug introduced in D62648, where Clang could infinite loop
if it became stuck on a single TypoCorrection when it was supposed to
be testing ambiguous corrections. Although not a common case, it could
happen if there are multiple possible corrections with the same edit
The fix is simply to wipe the TypoExpr from the TransformCache so that
the call to TransformTypoExpr doesn't use the CachedEntry.
Differential Revision: https://reviews.llvm.org/D67515