Driver/Darwin: When invoking the linker, automatically pass -object_path_lto so

that the linker has a place to put the temporary object file and can leave it
around (for the driver to clean up). This is important so that the object file
references in the debug info are preserved for possible use by dsymutil.
 - <rdar://problem/8294279> executable has no debug symbols when compiled with LTO

llvm-svn: 133543