Add an error message with fixit hint for changing '.' to '->'.

Add an error message with fixit hint for changing '.' to '->'.

This is mainly for attempting to recover in cases where a class provides
a custom operator-> and a '.' was accidentally used instead of '->' when
accessing a member of the object returned by the current object's
operator->.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@155580 91177308-0d34-0410-b5e6-96231b3b80d8