Determine the attribute subject for diagnostics based on declarative...

Determine the attribute subject for diagnostics based on declarative information in DeclNodes.td. This greatly reduces the number of enumerated values used for more complex diagnostics; these are now only required when the "attribute only applies to" diagnostic needs to be generated manually as part of semantic processing.

This also clarifies some terminology used by the diagnostic (methods -> Objective-C methods, fields -> non-static data members, etc).

Many of the tests needed to be updated in multiple places for the diagnostic wording tweaks. The first instance of the diagnostic for that attribute is fully specified and subsequent instances cut off the complete list (to make it easier if additional subjects are added in the future for the attribute).

llvm-svn: 319002