cc1as: consolidate option flags with cc1 and eliminate duplication

The clang -cc1as options are nearly a strict subset of -cc1. Instead of
duplicating the definitions and documentation, let's go ahead and share the
definitions in a similar way the current handling of combined driver and
frontend flags, eliminating some of the vestigial legacy surrounding the
assembler subcommand.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212620 91177308-0d34-0410-b5e6-96231b3b80d8