[NFC][Driver] Change MultilibBuilder interface

Decouple the interface of the MultilibBuilder flag method from how flags
are stored internally. Likewise change the addMultilibFlag function.

Currently a multilib flag like "-fexceptions" means a multilib is
*incompatible* with the -fexceptions command line option, which is
counter-intuitive. This change is a step towards changing this scheme.

Differential Revision: https://reviews.llvm.org/D151437