[LL FIR] KT-57314 Prepare for getting top-level names from index

- Getting top-level names from `declarationProvider` directly instead of
  from subordinate symbol providers has a lot of potential for
  performance, but the current performance of the index access is worse
  than the iterative version because scopes aren't optimized yet.
- This commit prepares `LLFirCombinedKotlinSymbolProvider` for the
  switch to getting top-level names from `declarationProvider`.

analysis/low-level-api-fir/src/org/jetbrains/kotlin/analysis/low/level/api/fir/providers/LLFirProviderHelper.kt

@@ -7,7 +7,7 @@ package org.jetbrains.kotlin.analysis.low.level.api.fir.providers
 
 import org.jetbrains.kotlin.analysis.low.level.api.fir.file.builder.LLFirFileBuilder
 import org.jetbrains.kotlin.analysis.low.level.api.fir.util.FirElementFinder
-import org.jetbrains.kotlin.analysis.low.level.api.fir.util.LLFirSymbolProviderNameCache
+import org.jetbrains.kotlin.analysis.low.level.api.fir.util.LLFirKotlinSymbolProviderNameCache
 import org.jetbrains.kotlin.analysis.providers.KotlinDeclarationProvider
 import org.jetbrains.kotlin.analysis.providers.KotlinPackageProvider
 import org.jetbrains.kotlin.builtins.StandardNames
@@ -60,15 +60,7 @@ internal class LLFirProviderHelper(
             }
         }
 
-    val symbolNameCache = object : LLFirSymbolProviderNameCache(firSession) {
-        override fun computeClassifierNames(packageFqName: FqName): Set<String>? =
-            declarationProvider
-                .getTopLevelKotlinClassLikeDeclarationNamesInPackage(packageFqName)
-                .mapTo(mutableSetOf()) { it.asString() }
-
-        override fun computeCallableNames(packageFqName: FqName): Set<Name>? =
-            declarationProvider.getTopLevelCallableNamesInPackage(packageFqName)
-    }
+    val symbolNameCache = LLFirKotlinSymbolProviderNameCache(firSession, declarationProvider)
 
     fun getFirClassifierByFqNameAndDeclaration(
         classId: ClassId,

analysis/low-level-api-fir/src/org/jetbrains/kotlin/analysis/low/level/api/fir/util/LLFirKotlinSymbolProviderNameCache.kt

@@ -0,0 +1,27 @@
+/*
+ * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.analysis.low.level.api.fir.util
+
+import org.jetbrains.kotlin.analysis.providers.KotlinDeclarationProvider
+import org.jetbrains.kotlin.fir.FirSession
+import org.jetbrains.kotlin.name.FqName
+import org.jetbrains.kotlin.name.Name
+
+/**
+ * An [LLFirSymbolProviderNameCache] that fetches top-level names from a Kotlin [declarationProvider].
+ */
+internal class LLFirKotlinSymbolProviderNameCache(
+    firSession: FirSession,
+    private val declarationProvider: KotlinDeclarationProvider,
+) : LLFirSymbolProviderNameCache(firSession) {
+    override fun computeClassifierNames(packageFqName: FqName): Set<String> =
+        declarationProvider
+            .getTopLevelKotlinClassLikeDeclarationNamesInPackage(packageFqName)
+            .mapTo(mutableSetOf()) { it.asString() }
+
+    override fun computeCallableNames(packageFqName: FqName): Set<Name> =
+        declarationProvider.getTopLevelCallableNamesInPackage(packageFqName)
+}