[FIR] Add ability to generate members and nested classifiers for existing classes

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

@@ -7,6 +7,7 @@ package org.jetbrains.kotlin.fir.scopes
 
 import org.jetbrains.kotlin.fir.resolve.substitution.ConeSubstitutor
 import org.jetbrains.kotlin.fir.symbols.impl.FirClassifierSymbol
+import org.jetbrains.kotlin.fir.symbols.impl.FirConstructorSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirNamedFunctionSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirVariableSymbol
 import org.jetbrains.kotlin.name.Name
@@ -37,6 +38,20 @@ class FirCompositeScope(val scopes: Iterable<FirScope>) : FirContainingNamesAwar
         }
     }
 
+    private inline fun <T> processComposite(
+        process: FirScope.((T) -> Unit) -> Unit,
+        noinline processor: (T) -> Unit
+    ) {
+        val unique = mutableSetOf<T>()
+        for (scope in scopes) {
+            scope.process {
+                if (unique.add(it)) {
+                    processor(it)
+                }
+            }
+        }
+    }
+
     override fun processFunctionsByName(name: Name, processor: (FirNamedFunctionSymbol) -> Unit) {
         return processComposite(FirScope::processFunctionsByName, name, processor)
     }
@@ -45,6 +60,10 @@ class FirCompositeScope(val scopes: Iterable<FirScope>) : FirContainingNamesAwar
         return processComposite(FirScope::processPropertiesByName, name, processor)
     }
 
+    override fun processDeclaredConstructors(processor: (FirConstructorSymbol) -> Unit) {
+        processComposite(FirScope::processDeclaredConstructors, processor)
+    }
+
     override fun getCallableNames(): Set<Name> {
         return scopes.flatMapTo(hashSetOf()) { it.getContainingCallableNamesIfPresent() }
     }