[analyzer] Fix Objective-C accessor body farms after 2073dd2d.
Fix a canonicalization problem for the newly added property accessor stubs that
was causing a wrong decl to be used for 'self' in the accessor's body farm.
Fix a crash when constructing a body farm for accessors of a property
that is declared and @synthesize'd in different (but related) interfaces.
Differential Revision: https://reviews.llvm.org/D70158