[lld-macho] Fix -bitcode_process_mode arg type

This is still undocumented and unsupported, but if someone passed it
before you would end up with a missing file error since this takes an
argument that wouldn't be handled.

Differential Revision: https://reviews.llvm.org/D130606

Author: keith

lld/MachO/Options.td

@@ -1182,7 +1182,7 @@ def allow_simulator_linking_to_macosx_dylibs : Flag<["-"], "allow_simulator_link
     HelpText<"This option is undocumented in ld64">,
     Flags<[HelpHidden]>,
     Group<grp_undocumented>;
-def bitcode_process_mode : Flag<["-"], "bitcode_process_mode">,
+def bitcode_process_mode : Separate<["-"], "bitcode_process_mode">,
     HelpText<"This option is undocumented in ld64">,
     Flags<[HelpHidden]>,
     Group<grp_undocumented>;