Give a description of the Objective-C garbage collection module flags.

The rule governing the flags is this:

  no-gc + no-gc   = no-gc
  no-gc +    gc   = no-gc
  no-gc + gc-only = error
     gc +    gc   = gc
     gc + gc-only = gc-only
gc-only + gc-only = gc-only

llvm-svn: 150646