[Driver] Enable selecting multiple multilibs

This will enable layering multilibs on top of each other.
For example a multilib containing only a no-exceptions libc++ could be
layered on top of a multilib containing C libs. This avoids the need
to duplicate the C library for every libc++ variant.

This change doesn't expose the functionality externally, it only opens
the functionality up to be potentially used by ToolChain classes.

Differential Revision: https://reviews.llvm.org/D143059