Implement the feature I was originally driving toward when I started

this saga. Teach the driver to detect a GCC installed along side Clang
using the existing InstalledDir support in the Clang driver. This makes
a lot of Clang's behavior more automatic when it is installed along side
GCC.

Also include the first test cases (more to come, honest) which test both
the install directory behavior, and the version sorting behavior to show
that we're actually searching for the best candidate GCC installation
now.

llvm-svn: 141145