Implement reasonable conversion ranking for Objective-C pointer

conversions (<rdar://problem/8592139>) for overload resolution. The
conversion ranking mirrors C++'s conversion ranking fairly closely,
except that we use a same pseudo-subtyping relationship employed by
Objective-C pointer assignment rather than simple checking
derived-to-base conversions. This change covers:

  - Conversions to pointers to a specific object type are better than
  conversions to 'id', 'Class', qualified 'id', or qualified 'Class'
  (note: GCC doesn't perform this ranking, but it matches C++'s rules
  for ranking conversions to void*).
  - Conversions to qualified 'id' or qualified 'Class' are better than
  conversions to 'id' or 'Class', respectively.
  - When two conversion sequences convert to the same type, rank the
  conversions based on the relationship between the types we're
  converting from. 
  - When two conversion sequences convert from the same non-id,
  non-Class type, rank the conversions based on the relationship of
  the types we're converting to. (note: GCC allows this ranking even
  when converting from 'id', which is extremeley dangerous).

llvm-svn: 124591