SR-14635: __SwiftValue should be transparent to casting

This PR changes the casting machinery to avoid casting `__SwiftValue` boxes
directly.  This forces the caster to instead unwrap `__SwiftValue` boxes and
retry with the inner content.  This results in boxed values being cast like the
inner content.

This fixes the behavior in situations like the following:
```
   let t = ...
   let s = t as Any as! AnyObject
   // `s` is now a `__SwiftValue` box
   // Next line should be true iff t conforms to NSCopying
   // Prior to this change, it always succeeds
   s is NSCopying
```

After this change, the above cast succeeds only if `t` actually
conforms to `NSCopying`.

This is a follow-on to PR#37683.
Related to: SR-14635

stdlib/public/runtime/DynamicCast.cpp

@@ -425,6 +425,29 @@ tryCastUnwrappingObjCSwiftValueSource(
       destFailureType, srcFailureType,
       /*takeOnSuccess=*/ false, mayDeferChecks);
 }
+#else
+static DynamicCastResult
+tryCastUnwrappingSwiftValueSource(
+  OpaqueValue *destLocation, const Metadata *destType,
+  OpaqueValue *srcValue, const Metadata *srcType,
+  const Metadata *&destFailureType, const Metadata *&srcFailureType,
+  bool takeOnSuccess, bool mayDeferChecks)
+{
+    const Metadata *srcInnerType;
+    const OpaqueValue *srcInnerValue;
+    if (swift_unboxFromSwiftValueWithType(srcValue, &srcInnerValue, &srcInnerType)) {
+      return DynamicCastResult::SuccessViaCopy;
+    }
+
+    // Note: We never `take` the contents from a SwiftValue box as
+    // it might have other references.  Instead, let our caller
+    // destroy the reference if necessary.
+    return tryCast(
+      destLocation, destType,
+      const_cast<OpaqueValue *>(srcInnerValue), srcInnerType,
+      destFailureType, srcFailureType,
+      /*takeOnSuccess=*/ false, mayDeferChecks);
+}
 #endif
 
 /******************************************************************************/
@@ -1474,6 +1497,16 @@ tryCastToClassExistential(
   }
 
   case MetadataKind::ObjCClassWrapper:
+#if SWIFT_OBJC_INTEROP
+    id srcObject;
+    memcpy(&srcObject, srcValue, sizeof(id));
+    if (getAsSwiftValue(srcObject) != nullptr) {
+      // Do not directly cast a `__SwiftValue` box
+      // Return failure so our caller will unwrap and try again
+      return DynamicCastResult::Failure;
+    }
+#endif
+    SWIFT_FALLTHROUGH;
   case MetadataKind::Class:
   case MetadataKind::ForeignClass: {
     auto srcObject = getNonNullSrcObject(srcValue, srcType, destType);
@@ -2285,8 +2318,11 @@ tryCast(
   case MetadataKind::Class: {
 #if !SWIFT_OBJC_INTEROP
     // Try unwrapping native __SwiftValue implementation
-    if (swift_unboxFromSwiftValueWithType(srcValue, destLocation, destType)) {
-      return DynamicCastResult::SuccessViaCopy;
+    auto subcastResult = tryCastUnwrappingSwiftValueSource(
+      destLocation, destType, srcValue, srcType,
+      destFailureType, srcFailureType, takeOnSuccess, mayDeferChecks);
+    if (isSuccess(subcastResult)) {
+      return subcastResult;
     }
 #endif
     break;