[Sema] Fix infinite typo correction loop.

NumTypos guard value ~0U doesn't prevent from creating new delayed typos. When
you create new delayed typos during typo correction, value ~0U wraps around to
0. When NumTypos is 0 we can miss some typos and treat an expression as it can
be typo-corrected. But if the expression is still invalid after correction, we
can get stuck in infinite loop trying to correct it.

Fix by not using value ~0U so that NumTypos correctly reflects the number of
typos.

rdar://problem/38642201

Reviewers: arphaman, majnemer, rsmith

Reviewed By: rsmith

Subscribers: rsmith, nicholas, cfe-commits

Differential Revision: https://reviews.llvm.org/D47341

llvm-svn: 335638