Clean up some cruft in the library path searching logic by making

'libdir' mean the actual library directory, not the GCC subdirectory of
the library directory. That was just a confusing pattern. Instead,
supply proper GCC subdirectories when scanning for various triple-based
subdirectories with a GCC installation in them. This also makes it much
more obvious how multiarch installations, which have a triple-based
prefix as well as suffix work.

Also clean up our handling of these triple-prefixed trees by using them
in both a multiarch pattern and a non-multiarch pattern whenever they
exist.

Note that this *does not* match what GCC does on Debian, the only truly
multiarch installation I've been able to get installed and test on. GCC
appears to have a bug, and ends up searching paths like
'/lib/../../lib32' which makes no sense what-so-ever. Instead, I've
tried to encode the rational logic that seems clearly intended by GCC's
pattern. GCC ends up with patterns like:

  /lib/../../lib32
  /usr/lib/../../lib32
  /usr/lib/x86_64-linux-gnu/../..lib32

Only the last one makes any sense having a '/../..' in it, so in Clang,
that's the only one which gets a '/../..' in it.

I *think* this will fix Debian multiarch links. I'm committing without
baking this logic into our test suite so I can test on a few different
systems. If all goes well (and no one screams) I'll check in some more
comprehensive tests for multiarch behavior tomorrow.

llvm-svn: 142133