Options: Use erase_if to remove Args from the list.

While there make getOption return a const reference so we don't have to put it
on the stack when calling methods on it. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209088 91177308-0d34-0410-b5e6-96231b3b80d8