[FIR] Make diagnostics collectors returns List instead of Iterable

idea/idea-frontend-fir/idea-fir-low-level-api/src/org/jetbrains/kotlin/idea/fir/low/level/api/diagnostics/AbstractFirIdeDiagnosticsCollector.kt

@@ -56,7 +56,7 @@ internal abstract class AbstractFirIdeDiagnosticsCollector(
         checkCanceled()
     }
 
-    override fun getCollectedDiagnostics(): Iterable<FirDiagnostic<*>> {
+    override fun getCollectedDiagnostics(): List<FirDiagnostic<*>> {
         // Not necessary in IDE
         return emptyList()
     }