Add category test case for attribute 'objc_protocol_requires_explicit_implementation'.

This test also illustrates that checking for properties is not working properly with
this attribute, as we are missing a diagnostic for a property not being implemented.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202335 91177308-0d34-0410-b5e6-96231b3b80d8