swift234

Implement support for Clang's objc_runtime_visible attribute.

When a Clang-defined Objective-C class has the objc_runtime_visible
attribute, use objc_lookUpClass to get the Objective-C class object
rather than referencing the symbol directly. Also, ban subclassing of
Objective-C-runtime-visible classes as well as @objc on members of
extensions of such classes.

As a drive-by needed for this test, make
ClassDecl::getObjCRuntimeName() respect the Clang objc_runtime_name
attribute.

Fixes rdar://problem/25494454.