FIR checker: add diagnostics for backing fields

compiler/testData/diagnostics/tests/backingField/ExtensionProperty.fir.kt

@@ -1,17 +0,0 @@
-// See KT-9303: synthetic field variable does not exist for extension properties
-val String.foo: Int
-    get() {
-        // No shadowing here
-        val field = 42
-        return field
-    }
-
-val String.bar: Int = 13
-    // Error
-    get() = <!UNRESOLVED_REFERENCE!>field<!>
-
-class My {
-    val String.x: Int = 7
-        // Error
-        get() = <!UNRESOLVED_REFERENCE!>field<!>
-}

compiler/testData/diagnostics/tests/backingField/ExtensionProperty.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // See KT-9303: synthetic field variable does not exist for extension properties
 val String.foo: Int
     get() {

compiler/testData/diagnostics/tests/backingField/FieldInInterface.fir.kt

@@ -1,4 +0,0 @@
-interface My {
-    val x: Int = <!PROPERTY_INITIALIZER_IN_INTERFACE!>0<!>
-        get() = field
-}

compiler/testData/diagnostics/tests/backingField/FieldInInterface.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 interface My {
     <!BACKING_FIELD_IN_INTERFACE!>val x: Int<!> = <!PROPERTY_INITIALIZER_IN_INTERFACE!>0<!>
         get() = field