Add upcoming feature ImplicitOpenExistentials for SE-0352

To maintain source compatibility, SE-0352 does not open existentials
with "self-conforming" type, such as `any Error` or existentials based
on `@objc` protocols. The proposal specified that this behavior would
change in Swift 6. Implement that behavior change, which can be
enabled prior to Swift 6 with the upcoming feature
`ImplicitOpenExistentials` (as documented in SE-0362).

Fixes #70873 / rdar://120902975.

test/Constraints/opened_existentials_feature.swift

@@ -0,0 +1,20 @@
+// RUN: %target-typecheck-verify-swift -enable-upcoming-feature ImplicitOpenExistentials
+// RUN: %target-typecheck-verify-swift -swift-version 6
+
+#if _runtime(_ObjC)
+@objc
+protocol X {}
+
+func foo<T: X>(_ val: T.Type) {}
+
+func bar(_ val: X.Type) {
+  // Only succeeds when we're allowed to open an @objc existential.
+  foo(val)
+}
+#endif
+
+func takeError<E: Error>(_ error: E) { }
+
+func passError(error: any Error) {
+  takeError(error)  // okay
+}