Add a comment explaining that I have a better plan for implementing the

GCC installation search that requires fewer filesystem operations.
Planning to implement that next as the current approcah while thorough
(and so far looks correct) does a very unfortunate number of filesystem
operations.

I'm motivated to fix this in no small part because I would like to
support a much larger space of triples and GCC versions, which would
explode the current algorithm.

llvm-svn: 141073