Sema: Diagnose opaque types that depend on availability of a custom domain.

Serialization and IRGen don't yet support opaque return types that would depend
on querying availability of a custom domain so we need to reject this code to
avoid mis-compiling it.

test/Availability/availability_custom_domains.swift

@@ -193,3 +193,19 @@ struct EnabledDomainAvailable {
     unavailableInDisabledDomain() // expected-error {{'unavailableInDisabledDomain()' is unavailable}}
   }
 }
+
+protocol P { }
+
+@available(EnabledDomain)
+struct AvailableInEnabledDomain: P { }
+
+@available(EnabledDomain, unavailable)
+struct UnavailableInEnabledDomain: P { }
+
+func testOpaqueReturnType() -> some P {
+  if #available(EnabledDomain) { // expected-error {{opaque return type cannot depend on EnabledDomain availability}}
+    return AvailableInEnabledDomain()
+  } else {
+    return UnavailableInEnabledDomain()
+  }
+}