swift234

Implement @nonobjc attribute

The following declaration kinds can be marked with this attribute:
- method
- property
- property accessor
- subscript
- constructor

Use cases include resolving circularity for bridging methods in an @objc
class, and allowing overloading methods and constructors in an @objc class
by signature by marking some of them @nonobjc.

It is an error to override an @objc method with a @nonobjc method. The
converse, where we override a @nonobjc method with a @objc method, is
explicitly supported.

It is also an error to put a @nonobjc attribute on a method which is
inferred as @objc due to being part of an @objc protocol conformance.

Fixes <rdar://problem/16763754>.

Swift SVN r28126