XFAIL prefer-debug-over-eh-frame.test on darwin

debug_frame does not seem to work on darwin, so there is nothing to
prefer.

Adding `-g` to the compiler command line is enough to get the
__debug_frame section added to the dsym file. Though lldb then finds the
section, and correctly assigns the section type to it, this does not
seem to be enough to get lldb to actually use this section for
unwinding.

llvm-svn: 361760