Option parsing: remove non-SUPPORT_ALIASARGS fall-back

The clients of this code have been updated to all support AliasArgs.

This depends on Clang r187538 and lld r187541.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187546 91177308-0d34-0410-b5e6-96231b3b80d8