This adds the most rudimentary support for linking in the Carbon driver
by calling out to the Clang driver and letting it do the linking. This
is a super minimal version to start with, just enough to be somewhat
useful and to exercise calling into Clang for this.
Also adds a Clang runner to help run the Clang driver. This is a bit
more complete, although it sill needs some significant work. For
example, it will need some significant enhancements to be able to
compile C++ code, etc.
One thing that will likely change here is to better manage streams and
work better as a library. For now, this is a bit limited because Clang
and LLVM don't provide the necessary support for this.
