Add FirScope::getCallableNames function

compiler/fir/tree/src/org/jetbrains/kotlin/fir/scopes/FirCompositeScope.kt

@@ -44,4 +44,8 @@ class FirCompositeScope(private val scopes: Iterable<FirScope>) : FirScope() {
     override fun processPropertiesByName(name: Name, processor: (FirVariableSymbol<*>) -> Unit) {
         return processComposite(FirScope::processPropertiesByName, name, processor)
     }
+
+    override fun getCallableNames(): Set<Name> {
+        return scopes.flatMapTo(mutableSetOf()) { it.getCallableNames() }
+    }
 }

compiler/fir/tree/src/org/jetbrains/kotlin/fir/scopes/FirScope.kt

@@ -34,6 +34,7 @@ abstract class FirScope {
 
     open fun mayContainName(name: Name) = true
 
+    open fun getCallableNames(): Set<Name> = emptySet()
 }
 
 fun FirTypeScope.processOverriddenFunctionsAndSelf(