Add a stop gap to Sema::CorrectTypo() to correct only up to 20 typos.

This is to address a serious performance problem observed when running
'clang -fsyntax-only' on really broken source files.  In one case,
repeatedly calling CorrectTypo() caused one source file to be rejected
after 2 minutes instead of 1 second.

This patch causes typo correction to take neglible time on that file
while still providing correction results for the first 20 cases.  I
felt this was a reasonable number for moderately broken source files.

I don't claim this is the best solution.  Comments welcome.  It is
necessary for us to address this issue because it is a serious
performance problem.

llvm-svn: 95049