Move the extern "C" sema tests to a new file.
Objective-C: Issue warning in couple of obscure cases
when property autosynthesis does not synthesize a property.
When property is declared 'readonly' in a super class and
is redeclared 'readwrite' in a subclass. When a property
autosynthesis causes it to share 'ivar' with another property.
// rdar://13388503