Add hooks for typo correction to ExternalSemaSource, courtesy of Luke Zarko.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@188196 91177308-0d34-0410-b5e6-96231b3b80d8
Forgot to add unittests/Sema/ before committing r188196 :(

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@188197 91177308-0d34-0410-b5e6-96231b3b80d8