Diagnose TypoExprs in a couple of error cases in ParsePostfixExpressionSuffix.

Also have CorrectDelayedTyposInExpr check that the Expr* isn't null
before trying to access its members. Fixes PR21679.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223162 91177308-0d34-0410-b5e6-96231b3b80d8