Log a warning if Obj-C -hash is called on a Swift type that is Equatable but not Hashable

The only reasonable way to handle this can lead to severe
performance drop-offs.  We should make sure the
user is aware of this.

Author: tbkka

stdlib/public/runtime/SwiftObject.mm

@@ -396,6 +396,11 @@ - (NSUInteger)hash {
       swift_conformsToProtocolCommon(
 	selfMetadata, &equatable_support::EquatableProtocolDescriptor));
   if (equatableConformance != nullptr) {
+    const char *clsName = class_getName([self class]);
+    warning(0,
+	    "Obj-C `-hash` invoked on a Swift object of type %s that is not Hashable; "
+	    "this can lead to severe performance problems",
+	    clsName);
     return (NSUInteger)1;
   }