[FIR] FirScriptConfigurationExtensionImpl: change source element for result$$ property

We should use the source from replaced `FirAnonymousInitializer` as it
is more equivalent. Another point – the previous implementation
duplicated the source between generated FirProperty and initializer

^KT-65344

analysis/low-level-api-fir/testData/getOrBuildFir/calls/syntheticResultDeclaration.txt

@@ -1,10 +1,9 @@
 KT element: KtCallExpression
-FIR element: FirPropertyImpl
+FIR element: FirFunctionCallImpl
 FIR source kind: KtRealSourceElementKind
 
 FIR element rendered:
-public final [ResolvedTo(BODY_RESOLVE)] val $$result: R|kotlin/Int| = R|/foo|()
-    public [ResolvedTo(BODY_RESOLVE)] get(): R|kotlin/Int|
+R|/foo|()
 
 FIR FILE:
 FILE: [ResolvedTo(IMPORTS)] syntheticResultDeclaration.kts