Fix parsing variable declaration in when subject

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

compiler/testData/psi/WhenWithSubjectVariable.kt

@@ -0,0 +1,14 @@
+fun test() {
+    when (val x1 = foo) {}
+    when (val x2t: T) {}
+    when (val y2t: T = foo) {}
+    when (val (x3, x4) = foo) {}
+    when (val (y3t: T, y4) = foo) {}
+    when (val (z3, z4t: T) = foo) {}
+    when (val (w3t: T, w4t: T) = foo) {}
+    when (val T.x5 = foo) {}
+    when (val T1.x5t: T2 = foo) {}
+    when (@Ann val x6a = foo) {}
+    when (val x7a: @Ann T = foo) {}
+    when (val x8a = @Ann foo) {}
+}