Refactor (with some rewriting) the DynamicLoaderMacOSX plugin into

a base class and a derived class, with the derived class containing
the methods specific to reading dyld's all_image_infos, dyld's
method of specifying images that have been loaded or unloaded, the
place where we put a breakpoint in dyld to get notified about newly
loaded or unloaded images.

This is in preparation for a second derived class which will use
some alternate methods for getting this information; that will be
a separate commit in the next few days.

There's a couple of ivars that should probably be in the derived
DyanmicLoaderMacOSX class instead of the base DynamicLoaderDarwin
class (m_dyld_image_infos, m_dyld_image_infos_stop_id).  I don't
think I'll need to use these in the new derived class - I'll 
move them down to DynamicLoaderMacOSX if it works out that way;
it'll simplify locking if I can do that.

<rdar://problem/25251243> 

llvm-svn: 275733