Change parsing rules for primary constructor

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

compiler/testData/psi/primaryConstructor/nestedClassAmbiguity.kt

@@ -0,0 +1,28 @@
+class Outer1 {
+    class Nested1
+
+    private Ann constructor()
+}
+
+class Outer2 {
+    class Nested2;
+
+    private Ann constructor()
+}
+
+class Outer3 {
+    class Nested3
+
+    private Ann constructor() : super() {}
+}
+
+class Outer4 {
+    object Nested4
+    constructor() {}
+
+    object Nested5 private constructor() : super() {}
+}
+
+object TopLevel constructor(val x: Int) {
+    fun foo() {}
+}