Invert the loop for detecting installed GCC trees. This make the loop

find the newest GCC available, among other goodness. It makes the entire
system much less prone to error from prefixes and/or system roots
pruning early the set of triples and GCC versions available.

Also, improve some comments and simplify the forms of some of the loops.

This causes the driver to stat directories more often than is strictly
necessary, but the alternatives which I looked at that still
accomplished this goal needed quite a bit more code and were likely not
much faster.

Test cases for this, now that our behavior here is significantly more
principled and predictable, should come tomorrow as I walk back through
VMs looking for edge cases that are missed after this.

llvm-svn: 141072