Refactor the detection of a GCC installation into a helper class. This

is designed to allow the detection to record more rich information about
the installation than just a single path.

Mostly, the functionality remains the same. This is primarily
a factoring change. However, the new factoring immediately fixes one
issue where on ubuntu we didn't walk up enough layers to reach the
parent lib path. I'll have a test tree for that once I finish making the
Ubuntu tree work reasonably.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@141011 91177308-0d34-0410-b5e6-96231b3b80d8