Sema: Suppress explicit availability diagnostics on non-Darwin targets.

Suggesting the addition of an `@available` attribute on Linux and Windows is
inapprorpriate given that Swift is not ABI stable on those platforms.

Resolves rdar://107387133

include/swift/AST/ASTContext.h

@@ -970,6 +970,9 @@ public:
   /// Swift compiler for future versions of the target platform.
   AvailabilityContext getSwiftFutureAvailability();
 
+  /// Returns `true` if versioned availability annotations are supported for the
+  /// target triple.
+  bool supportsVersionedAvailability() const;
 
   //===--------------------------------------------------------------------===//
   // Diagnostics Helper functions