[test] Add a test for SR-8493, so we don't regress (#19068)

https://bugs.swift.org/browse/SR-8493

Author: jrose-apple

validation-test/Sema/Inputs/sr8493-helper.swift

@@ -0,0 +1,12 @@
+class StateAnimatorGroup<State> {
+    func add<T: StateAnimator>(_ animator: T) where T.State == State {}
+    func set(_ state: State) {}
+}
+
+extension StateAnimatorGroup: StateAnimator where State == String {}
+
+protocol StateAnimator {
+    associatedtype State
+
+    func set(_ state: State)
+}

validation-test/Sema/sr8493.swift

@@ -0,0 +1,5 @@
+// RUN: %empty-directory(%t)
+// RUN: cp %s %t/main.swift
+// RUN: %target-swift-frontend -primary-file %t/main.swift %S/Inputs/sr8493-helper.swift -emit-ir -o /dev/null
+
+StateAnimatorGroup<String>().set("")