[FIR] Consider getter and setter for CANNOT_WEAKEN_ACCESS_PRIVILEGE

Fill source with correct value instead of null
for FirDefaultPropertySetter in light tree converter

compiler/testData/diagnostics/tests/OverridenSetterVisibility.fir.kt

@@ -1,29 +0,0 @@
-public interface ITest {
-    public var prop : Int
-        get() = 12
-        set(value) {}
-}
-
-abstract class ATest {
-    protected open var prop2 : Int
-        get() = 13
-        set(value) {}
-}
-
-class Test: ATest(), ITest {
-    override var prop : Int
-        get() = 12
-        private set(value) {}
-
-    override var prop2 : Int
-        get() = 14
-        <!SETTER_VISIBILITY_INCONSISTENT_WITH_PROPERTY_VISIBILITY!>internal<!> set(value) {}
-}
-
-fun main() {
-    val test = Test()
-    <!INVISIBLE_SETTER!>test.prop<!> = 12
-
-    val itest: ITest = test
-    itest.prop = 12 // No error here
-}