Issue Description:

When running lit tests from the command line in an Xcode build directory, you have to provide the build_mode parameter in order for our lit.cfg to find the correct built binaries (see comments on SR-7562). lit.cfg itself should be able to detect if it's being run from an Xcode-based build directory and complain if build_mode isn't set.

I'm not exactly sure of the best way to do this (possibly another expansion in lit.site.cfg.in based on the CMake generator?) but it would be a nice usability improvement.