[analyzer] Do not try to body-farm Objective-C properties with custom accessors.

If a property is defined with a custom getter, we should not behave as if
the getter simply returns an instance variable. We don't support setters,
so they aren't affected.

On top of being the right thing to do, this also fixes a crash on
the newly added test - in which a property and its getter are defined
in two separate categories.

rdar://problem/47051544

Differential Revision: https://reviews.llvm.org/D56823

llvm-svn: 351609