Don't allow overloading by opaque types with different constraints.

The mangling unfortunately doesn't take the constraints into account, leading to
SR-10725 | rdar://problem/50987172. Changing the mangling at this point is hazardous, so it's probably best
to disallow this for now.

lib/AST/Decl.cpp

@@ -2369,7 +2369,7 @@ static Type mapSignatureFunctionType(ASTContext &ctx, Type type,
     // Functions and subscripts cannot overload differing only in opaque return
     // types. Replace the opaque type with `Any`.
     if (auto opaque = type->getAs<OpaqueTypeArchetypeType>()) {
-      type = opaque->getExistentialType();
+      type = ProtocolCompositionType::get(ctx, {}, /*hasAnyObject*/ false);
     }
 
     return mapSignatureParamType(ctx, type);