Revert "Remove the darwin gdb option, that version of gdb is now dead and the...

Revert "Remove the darwin gdb option, that version of gdb is now dead and the rest of the compatibility should be done on a dwarf-N level."

This reverts commit r189903.

This commit broke the phase 1 buildbot for a while.

http://lab.llvm.org:8013/builders/clang-x86_64-darwin11-nobootstrap-RAincremental/builds/6684

llvm-svn: 189913