Add initial callback object support to Sema::CorrectTypo.

Add initial callback object support to Sema::CorrectTypo.

Also includes two examples of the callback: a wrapper/replacement for
the CorrectTypoContext enum, and a conversion of the two calls to
CorrectTypo in SemaDeclCXX.cpp (one of which provides verifiable
improvement to the typo correction, as demonstrated in the added test).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147962 91177308-0d34-0410-b5e6-96231b3b80d8