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

Make the suffixes optional everywhere, and just make sure they have the
right value. The suffixes aren't the interesting part of this test
anyways.

Sorry for the churn as I let the bots try out various patterns.

llvm-svn: 140927

run it.

llvm-svn: 140925

llvm-svn: 140920

left a FIXME to go track down more distros and see if 'lib' is ever the
64-bit half.

This should hopefully appease the build bots.

llvm-svn: 140915

llvm-svn: 140913

invocations on Linux.

llvm-svn: 140909