Significantly fix Clang's header search for Ubuntu (and possibly other
modern Debian-based distributions) due to on-going multiarch madness.

It appears that when the multiarch heeader search support went into the
clang driver, it went in in a quite bad state. The order of includes
completely failed to match the order exhibited by GCC, and in a specific
case -- when the GCC triple and the multiarch triple don't match as with
i686-linux-gnu and i386-linux-gnu -- we would absolutely fail to find
the libstdc++ target-specific header files.

I assume that folks who have been using Clang on Ubuntu 32-bit systems
have been applying weird patches to hack around this. I can't imagine
how else it could have worked. This was originally reported by a 64-bit
operating system user who had a 32-bit crosscompiler installed. We tried
to use that rather than the bi-arch support of the 64-bit compiler, but
failed due to the triple differences.

I've corrected all the wrong orderings in the existing tests and added
a specific test for the multiarch triple strings that are different in
a significant way. This should significantly improve the usability of
Clang when checked out vanilla from upstream onto Ubuntu machines with
an i686 GCC installation for whatever reason.