Initial cut at integrating compiler-rt (on Darwin) w/ clang build.

 - compiler-rt should be checked out into $LLVM_SRC_ROOT/projects/compiler-rt.

 - On Darwin, this will automatically build the runtime libraries clang needs
   into $OBJROOT/lib/clang/<version>/darwin/...

 - The mechanism can easily support other platforms, and can eventually support
   multiple platforms once clang has some kind of configure process (for
   specifying the desired targets).

 - Feedback on the approach is welcome.

llvm-svn: 93910