[LL FIR] KT-57220 Make combined symbol provider constructors private

- The combined symbol provider should always be constructed using the
  dedicated `merge` function.

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

@@ -40,7 +40,7 @@ import org.jetbrains.kotlin.psi.KtFile
  *
  * [declarationProvider] must have a scope which combines the scopes of the individual [providers].
  */
-internal class LLFirCombinedKotlinSymbolProvider(
+internal class LLFirCombinedKotlinSymbolProvider private constructor(
     session: FirSession,
     private val project: Project,
     private val providers: List<LLFirProvider.SymbolProvider>,

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

@@ -23,7 +23,7 @@ import org.jetbrains.kotlin.name.Name
  * heuristics are checked only once.
  */
 @OptIn(FirSymbolProviderInternals::class)
-internal class LLFirCombinedSyntheticFunctionSymbolProvider(
+internal class LLFirCombinedSyntheticFunctionSymbolProvider private constructor(
     session: FirSession,
     private val providers: List<FirSyntheticFunctionInterfaceProviderBase>,
 ) : FirSymbolProvider(session) {