FIR: implement backing field references via 'field' synthetic variable

compiler/fir/resolve/testData/resolve/stdlib/backingField.kt

@@ -0,0 +1,7 @@
+var myProperty = listOf(1, 2, 3)
+    get() {
+        return field + field
+    }
+    set(param) {
+        field = param
+    }

compiler/fir/resolve/testData/resolve/stdlib/backingField.txt

@@ -0,0 +1,8 @@
+FILE: backingField.kt
+    public final var myProperty: R|kotlin/collections/List<kotlin/Int>| = R|kotlin/collections/listOf|<R|kotlin/Int|>(Int(1), Int(2), Int(3))
+        public get(): R|kotlin/collections/List<kotlin/Int>| {
+            ^ F|/myProperty|.R|kotlin/collections/plus|<R|kotlin/Int|>(F|/myProperty|)
+        }
+        public set(param: R|kotlin/collections/List<kotlin/Int>|): R|kotlin/Unit| {
+            F|/myProperty| = R|<local>/param|
+        }

compiler/fir/resolve/testData/resolve/stdlib/hashSet.kt

@@ -0,0 +1,13 @@
+import java.util.HashSet
+
+val a: MutableSet<String>? = HashSet()
+
+var b: MutableSet<String>? = null
+    set(_) {
+        field = HashSet()
+    }
+
+fun foo() {
+    var c: MutableSet<String>? = null
+    c = HashSet()
+}

compiler/fir/resolve/testData/resolve/stdlib/hashSet.txt

@@ -0,0 +1,12 @@
+FILE: hashSet.kt
+    public final val a: R|kotlin/collections/MutableSet<kotlin/String>|? = R|java/util/HashSet.HashSet|<R|kotlin/Any|?>()
+        public get(): R|kotlin/collections/MutableSet<kotlin/String>|?
+    public final var b: R|kotlin/collections/MutableSet<kotlin/String>|? = Null(null)
+        public get(): R|kotlin/collections/MutableSet<kotlin/String>|?
+        public set(_: R|kotlin/collections/MutableSet<kotlin/String>|?): R|kotlin/Unit| {
+            F|/b| = R|java/util/HashSet.HashSet|()
+        }
+    public final fun foo(): R|kotlin/Unit| {
+        lvar c: R|kotlin/collections/MutableSet<kotlin/String>|? = Null(null)
+        R|<local>/c| = R|java/util/HashSet.HashSet|()
+    }