[analyzer] Add diagnostic text for generalized refcount annotations.

Add a 'Generalized' object kind to the retain-count checker and suitable
generic diagnostic text for retain-count diagnostics involving those objects.

For now the object kind is introduced in summaries by 'annotate' attributes.
Once we have more experience with these annotations we will propose explicit
attributes.

Patch by Malhar Thakkar!

Differential Revision: https://reviews.llvm.org/D35613

llvm-svn: 308990