Extract diagnostics about references to variables to a separate one

Note that I've left in FIR everything as is to avoid non-trivial
refactoring that is required right now to report more specific diagnostics

 ^KT-59152

compiler/testData/diagnostics/tests/callableReference/property/backingField.fir.kt

@@ -0,0 +1,7 @@
+// !DIAGNOSTICS: -UNUSED_EXPRESSION
+
+val i: Int = 10
+    get() {
+        ::<!UNSUPPORTED!>field<!>
+        return field
+    }