Warn that -O4 is the same as -O3.

We error on -O5 and higher. While it is tempting to do the same for -O4, I
agree with Jordan Rose: we should warn for a release at least first.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189369 91177308-0d34-0410-b5e6-96231b3b80d8