[FIR] FirSymbolNamesProvider: Provide default implementation for mayHaveSyntheticFunctionType

- Composite providers may call `mayHaveSyntheticFunctionType` on *all*
  of their children if *one* child has
  `mayHaveSyntheticFunctionTypes == true`, so
  `mayHaveSyntheticFunctionType` should have a proper default
  implementation to avoid runtime errors.
- The alternative of requiring to check `mayHaveSyntheticFunctionTypes`
  before calling `mayHaveSyntheticFunctionType` at the call-site was
  considered, but deemed too error-prone. Given that
  `mayHaveSyntheticFunctionType` is an optimization and admits false
  positives, an imprecise default implementation of it is allowed.

Thanks to Mikhail Glukhikh for discovering this problem.

compiler/fir/providers/src/org/jetbrains/kotlin/fir/resolve/providers/FirSymbolNamesProvider.kt

@@ -63,8 +63,7 @@ abstract class FirSymbolNamesProvider {
     /**
      * Whether [classId] is considered a generated function type within the provider's scope and session.
      */
-    open fun mayHaveSyntheticFunctionType(classId: ClassId): Boolean =
-        error("`mayHaveSyntheticFunctionType` needs to be implemented when `mayHaveSyntheticFunctionTypes` is true.")
+    open fun mayHaveSyntheticFunctionType(classId: ClassId): Boolean = mayHaveSyntheticFunctionTypes
 
     /**
      * Checks if the provider's scope may contain a top-level classifier (class, interface, object, or type alias) with the given [classId].