[Driver] Update handling of c++ and runtime directories

This is a follow up to r361432 and r361504 which addresses issues
introduced by those changes. Specifically, it avoids duplicating
file and runtime paths in case when the effective triple is the
same as the cannonical one. Furthermore, it fixes the broken multilib
setup in the Fuchsia driver and deduplicates some of the code.

Differential Revision: https://reviews.llvm.org/D62442

llvm-svn: 361709