Provide an NSUnsupported macro for not supported implementations (#1150)

When developing cross-platform applications, an `NSUnimplemented()`
is currently used to identify when a method is not and will not be
implemented.

Since `NSUnimplemented()` is used to also mean 'not yet implemented',
using a distinct name for the same functionality will allow the
two states to be distinguished, in the same way that
`NSRequiresConcreteImplementation()` is used to indicate that a
subtype must provide the behaviour.

Author: alblue

Foundation/NSObjCRuntime.swift

@@ -202,6 +202,13 @@ internal func NSUnimplemented(_ fn: String = #function, file: StaticString = #fi
     fatalError("\(fn) is not yet implemented", file: file, line: line)
 }
 
+internal func NSUnsupported(_ fn: String = #function, file: StaticString = #file, line: UInt = #line) -> Never {
+    #if os(Android)
+    NSLog("\(fn) is not supported on this platform. \(file):\(line)")
+    #endif
+    fatalError("\(fn) is not supported on this platform", file: file, line: line)
+}
+
 internal func NSInvalidArgument(_ message: String, method: String = #function, file: StaticString = #file, line: UInt = #line) -> Never {
     fatalError("\(method): \(message)", file: file, line: line)
 }