[Fuchsia] Support multilib for -fsanitize=address and -fno-exceptions

This introduces a support for multilibs to Fuchsia driver. Unlike the
existing multilibs that are used primarily for handling different
architecture variants, we use multilibs to handle different variants
of Clang runtime libraries: -fsanitize=address and -fno-exceptions
are the two we support initially. This replaces the existing support
for sanitized runtimes libraries that was only used by Fuchsia driver
and it also refactors some of the logic to allow sharing between GNU
and Fuchsia drivers.

Differential Revision: https://reviews.llvm.org/D61040

llvm-svn: 359360