[Driver] Change multilib selection algorithm

The new algorithm is:
1. Find all multilibs with flags that are a subset of the requested
   flags.
2. If more than one multilib matches, choose the last.

In addition a new selection mechanism is permitted via an overload of
MultilibSet::select() for which multiple multilibs are returned.
This allows layering multilibs on top of each other.

Since multilibs are now ordered within a list, they no longer need a
Priority field.

The new algorithm is different to the old algorithm, but in practise
the old algorithm was always used in such a way that the effect is the
same.
The old algorithm was to find the set intersection of the requested
flags (with the first character of each removed) with each multilib's
flags (ditto), and for that intersection check whether the first
character matched. However, ignoring the first characters, the
requested flags were always a superset of all the multilibs flags.
Therefore the new algorithm can be used as a drop-in replacement.

The exception is Fuchsia, which needs adjusting slightly to set both
fexceptions and fno-exceptions flags.

Differential Revision: https://reviews.llvm.org/D142905