Fix parsing variable declaration in when subject

- parse destructuring declarations
- parse annotations on variable declaration in when subject

compiler/testData/psi/WhenWithSubjectVariable_ERR.kt

@@ -0,0 +1,12 @@
+fun test() {
+    when (val) {}
+    when (val x1) {}
+    when (val x2 = ) {}
+    when (val x3: ) {}
+
+    when (fun foo() {}) {}
+    when (class C {}) {}
+    when (interface I {}) {}
+    when (object Obj {}) {}
+    when (typealias TA = T) {}
+}