[NFC][Driver] Change Multilib flag representation

This new representation means that a valid command line option may
potentially be used directly as a multilib flag without any translation.

To indicate that a flag is required not to be present, its first
character is replaced with '!', which is intended for consistency with
the logical not operator in many programming languages.

Reviewed By: simon_tatham

Differential Revision: https://reviews.llvm.org/D151438