FIR tree & builder: support expect / actual

compiler/testData/fir/rawBuilder/declarations/expectActual.kt

@@ -0,0 +1,11 @@
+expect class MyClass
+
+expect fun foo(): String
+
+expect val x: Int
+
+actual class MyClass
+
+actual fun foo() = "Hello"
+
+actual val x = 42

compiler/testData/fir/rawBuilder/declarations/expectActual.txt

@@ -0,0 +1,13 @@
+FILE: expectActual.kt
+    public? final? expect class MyClass() {
+    }
+    public? final? expect function foo(): String
+    public? final? expect property x(val): Int
+        public? get(): Int
+    public? final? actual class MyClass() {
+    }
+    public? final? actual function foo(): <implicit> {
+        STUB
+    }
+    public? final? actual property x(val): <implicit> = STUB
+        public? get(): <implicit>