[analyzer] Prefer accessor method in extension over category in CallEvent.

In ObjCMethodCall:getRuntimeDefinition(), if the method is an accessor in a
category, and it doesn't have a self declaration, first try to find the method
in a class extension. This works around a bug in Sema where multiple accessors
are synthesized for properties in class extensions that are redeclared in a
category. The implicit parameters are not filled in for the method on the
category, which causes a crash when trying to synthesize a getter for the
property in BodyFarm. The Sema bug is tracked as rdar://problem/25481164.

rdar://problem/25056531

llvm-svn: 265103