Improve understanding post increment and decrement.

Before: (a->f()) ++;
        a[42] ++;
After:  (a->f())++;
        a[42]++;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172400 91177308-0d34-0410-b5e6-96231b3b80d8