"field": deprecated warnings introduced, compiler tests migrated to the new syntax

compiler/testData/diagnostics/tests/controlFlowAnalysis/backingFieldInsideGetter.kt

@@ -5,7 +5,7 @@ import java.util.HashSet
 val a: MutableSet<String>? = null
     get() {
         if (a == null) {
-            $a = HashSet()
+            field = HashSet()
         }
         return a
     }
@@ -14,7 +14,7 @@ class R {
     val b: String? = null
         get() {
             if (b == null) {
-                $b = "b"
+                field = "b"
             }
             return b
         }

compiler/testData/diagnostics/tests/controlFlowAnalysis/kt1571.kt

@@ -19,7 +19,7 @@ class A() {
 val a : A = A()
 get() {
     c0++
-    return $a
+    return field
 }
 
 fun box() : String {

compiler/testData/diagnostics/tests/controlFlowAnalysis/kt2960.kt

@@ -29,7 +29,7 @@ class A {
       get() {
           val x : Int
           doSmth(<!UNINITIALIZED_VARIABLE!>x<!>)
-          return $a
+          return field
       }
 }