FIR checker: introduce top-level function checker

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/checkers/CommonDeclarationCheckers.kt

@@ -59,6 +59,10 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
         FirConstructorAllowedChecker,
     )
 
+    override val fileCheckers: Set<FirFileChecker> = setOf(
+        FirTopLevelFunctionChecker,
+    )
+
     override val controlFlowAnalyserCheckers: Set<FirControlFlowChecker> = setOf(
         FirCallsEffectAnalyzer,
         FirReturnsImpliesAnalyzer,