[LL FIR] LLFirProvider: move backing field check to helper function

^KT-59505

analysis/low-level-api-fir/src/org/jetbrains/kotlin/analysis/low/level/api/fir/providers/LLFirProvider.kt

@@ -89,7 +89,6 @@ internal class LLFirProvider(
 
         val fir = symbol.fir
         return when {
-            symbol is FirBackingFieldSymbol -> getFirCallableContainerFile(symbol.fir.propertySymbol)
             symbol is FirSyntheticPropertySymbol && fir is FirSyntheticProperty -> getFirCallableContainerFile(fir.getter.delegate.symbol)
             else -> {
                 symbol.callableId.classId?.let { SyntheticFirClassProvider.getInstance(session).getFirClassifierContainerFileIfAny(it) }

analysis/low-level-api-fir/src/org/jetbrains/kotlin/analysis/low/level/api/fir/util/containingFileUtils.kt

@@ -22,6 +22,7 @@ fun FirElementWithResolveState.getContainingFile(): FirFile? {
         is FirTypeParameter -> containingDeclarationSymbol.fir.getContainingFile()
         is FirPropertyAccessor -> propertySymbol.fir.getContainingFile()
         is FirValueParameter -> containingFunctionSymbol.fir.getContainingFile()
+        is FirBackingField -> propertySymbol.fir.getContainingFile()
         is FirCallableDeclaration -> provider.getFirCallableContainerFile(symbol)
         is FirClassLikeDeclaration -> provider.getFirClassifierContainerFileIfAny(symbol)
         is FirAnonymousInitializer -> containingClass().getContainingFile()