Change parsing rules for primary constructor

- Parse 'constructor' keyword before value arguments
- Allow unescaped annotations

compiler/testData/psi/primaryConstructor/valid.kt

@@ -0,0 +1,13 @@
+class A0
+constructor() {}
+class A1
+private constructor(y: Int) : Base1(), Base2 {
+    val x: Int
+}
+class A2 @private constructor(y: Int)
+
+class A3 Ann(1) private constructor(y: Int)
+
+class A4 private Ann(1) constructor(y: Int)
+
+class A5 @Ann private constructor() {}