ObjectiveC Class Properties: warn if a class property accessor is mistakenly an

instance method.

When diagnosing unimplemented class property, make sure we emit
a warning when we only see an instance method with the right selector.

Also warn when we only see a class method for an instance property.

rdar://26141719


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@269968 91177308-0d34-0410-b5e6-96231b3b80d8