[FIR] Fix substituted synthetic property with overridden getter only

When building the original for a substitution override for a synthetic
property, use the initial setter as fallback when unsubstituting it
returns null. This can happen when a generic class overrides the getter
of a synthetic property of a non-generic class. Then the setter is never
substituted, therefore there is nothing to unsubstitute.

^KT-57168 Fixed

compiler/fir/fir2ir/tests-gen/org/jetbrains/kotlin/test/runners/codegen/FirLightTreeBlackBoxCodegenTestGenerated.java

@@ -28933,6 +28933,12 @@ public class FirLightTreeBlackBoxCodegenTestGenerated extends AbstractFirLightTr
             runTest("compiler/testData/codegen/box/javaInterop/syntheticPropClashingWithJvmField.kt");
         }
 
+        @Test
+        @TestMetadata("syntheticPropOverriddenGetter.kt")
+        public void testSyntheticPropOverriddenGetter() throws Exception {
+            runTest("compiler/testData/codegen/box/javaInterop/syntheticPropOverriddenGetter.kt");
+        }
+
         @Test
         @TestMetadata("unresolvedJavaClassInDifferentFile.kt")
         public void testUnresolvedJavaClassInDifferentFile() throws Exception {

compiler/fir/fir2ir/tests-gen/org/jetbrains/kotlin/test/runners/codegen/FirPsiBlackBoxCodegenTestGenerated.java

@@ -28933,6 +28933,12 @@ public class FirPsiBlackBoxCodegenTestGenerated extends AbstractFirPsiBlackBoxCo
             runTest("compiler/testData/codegen/box/javaInterop/syntheticPropClashingWithJvmField.kt");
         }
 
+        @Test
+        @TestMetadata("syntheticPropOverriddenGetter.kt")
+        public void testSyntheticPropOverriddenGetter() throws Exception {
+            runTest("compiler/testData/codegen/box/javaInterop/syntheticPropOverriddenGetter.kt");
+        }
+
         @Test
         @TestMetadata("unresolvedJavaClassInDifferentFile.kt")
         public void testUnresolvedJavaClassInDifferentFile() throws Exception {

compiler/fir/providers/src/org/jetbrains/kotlin/fir/resolve/calls/Synthetics.kt

@@ -130,7 +130,7 @@ class FirSyntheticPropertiesScope private constructor(
             property.originalForSubstitutionOverrideAttr = buildSyntheticProperty(
                 propertyName,
                 it,
-                matchingSetter?.originalForSubstitutionOverride
+                matchingSetter?.originalForSubstitutionOverride ?: matchingSetter
             )
         }
         val syntheticSymbol = property.symbol