[FIR] Use stubs for creating lazy bodies of backing field initializer

Ninth step for ^KT-52615

Merge-request: KT-MR-8756
Merged-by: Egor Kulikov <Egor.Kulikov@jetbrains.com>

compiler/fir/raw-fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt

@@ -1705,7 +1705,7 @@ open class RawFirBuilder(
 
         private fun KtDeclarationWithInitializer.toInitializerExpression() =
             runIf(hasInitializer()) {
-                buildOrLazyExpression(initializer?.toFirSourceElement()) {
+                buildOrLazyExpression(null) {
                     initializer.toFirExpression("Should have initializer")
                 }
             }