Rework the selection of builtin library search paths on Linux to

precisely match the pattern and logic used by the GCC driver on Linux as
of a recent SVN checkout.

This happens to follow a *much* more principled approach. There is
a strict hierarchy of paths examined, first with multilib-suffixing,
second without such suffixing. Any and all of these directories which
exist will be added to the library search path when using GCC.

There were many places where Clang followed different paths, omitted
critical entries, and worst of all (in terms of challenges to debugging)
got the entries in a subtly wrong order.

If this breaks Clang on a distro you use, please let me know, and I'll
work with you to figure out what is needed to work on that distro. I've
checked the behavior of the latest release of Ubuntu, OpenSUSE, Fedora,
and Gentoo. I'll be testing it on those as well as Debian stable and
unstable and ArchLinux. I may even dig out a Slackware install.

No real regression tests yet, those will follow once I add enough
support for sysroot to simulate various distro layouts in the testsuite.

llvm-svn: 140981