Revert a change from r222797 that is no longer needed and can cause

Revert a change from r222797 that is no longer needed and can cause
infinite recursion.

Also guard against said infinite recursion by adding an assert that will
trigger if CorrectDelayedTyposInExpr is called before a previous call to
CorrectDelayedTyposInExpr returns (i.e. if the TreeTransform run by
CorrectDelayedTyposInExpr calls a sequence of methods that
end up calling CorrectDelayedTyposInExpr, as the new test case had done
prior to this commit). Fixes PR22292.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@227368 91177308-0d34-0410-b5e6-96231b3b80d8