Add diagnostic test for @Observable macro (#67250)

This adds a diagnostic test that validates errors for putting
the `@Observable` macro on a struct, enum, or actor.

Author: natecook1000

test/stdlib/Observation/ObservableDiagnostics.swift

@@ -0,0 +1,25 @@
+// REQUIRES: swift_swift_parser
+
+// RUN: %target-typecheck-verify-swift -disable-availability-checking -parse-as-library -enable-experimental-feature InitAccessors -enable-experimental-feature Macros -enable-experimental-feature ExtensionMacros -plugin-path %swift-host-lib-dir/plugins
+
+// Asserts is required for '-enable-experimental-feature InitAccessors'.
+// REQUIRES: asserts
+
+// REQUIRES: observation
+// REQUIRES: concurrency
+// REQUIRES: objc_interop
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+
+import Observation
+
+@Observable // expected-error {{'@Observable' cannot be applied to actor type 'MyActor'}}
+actor MyActor {}
+
+@Observable // expected-error {{'@Observable' cannot be applied to enumeration type 'MyEnum'}}
+enum MyEnum {
+  case myCase // required for macro invocation
+}
+
+@Observable // expected-error {{'@Observable' cannot be applied to struct type 'MyStruct'}}
+struct MyStruct {}