Instead of having -Os/-Oz add OptimizeForSize/MinSize first, and later

Instead of having -Os/-Oz add OptimizeForSize/MinSize first, and later
having OptimizeNone remove them again, just don't add them in the
first place if the function already has OptimizeNone.

Note that MinSize can still appear due to attributes on different
declarations; a future patch will address that.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224047 91177308-0d34-0410-b5e6-96231b3b80d8