Unify return type checking for functions and ObjC methods. Move all the

Unify return type checking for functions and ObjC methods.  Move all the
random checks for ObjC object return types to SemaType.cpp.

Fixes issue with ObjC method type checking reported on cfe-dev.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184006 91177308-0d34-0410-b5e6-96231b3b80d8