[LL FIR] KT-60570 Use emptySet in Kotlin symbol names provider to conserve memory

- As mentioned in KT-60570, memory consumption in cached symbol names
  providers is caused partially by lots of instances of empty mutable
  sets. This commit replaces such sets with the `emptySet()` singleton.
- This improvement also makes sense for callable names because the IDE
  "package names with top-level callables" set currently contains all
  package names in the project.
- In my local tests of the `setUp` Find Usages performance test, the
  retained size of `FirDelegatingCachedSymbolNamesProvider` is cut down
  from 208MB to 170MB by the empty class name set optimization, and from
  170MB to 115MB by the empty callable name set optimization.

analysis/analysis-internal-utils/src/org/jetbrains/kotlin/analysis/utils/collections/collectionUtils.kt

@@ -0,0 +1,16 @@
+/*
+ * 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.utils.collections
+
+/**
+ * Maps all elements of this non-empty collection with the given [transform] function to a new mutable set, or returns [emptySet] if this
+ * collection is empty.
+ *
+ * [mapToSet] should be preferred over `collection.mapTo(mutableSetOf()) { ... }` when `collection` may be empty and the resulting set may
+ * be cached, because [mapToSet] saves memory by avoiding the creation of an empty mutable set.
+ */
+public inline fun <T, R> Collection<T>.mapToSet(transform: (T) -> R): Set<R> =
+    if (isNotEmpty()) mapTo(mutableSetOf(), transform) else emptySet()

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

@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.analysis.low.level.api.fir.util
 
 import org.jetbrains.kotlin.analysis.providers.KotlinDeclarationProvider
+import org.jetbrains.kotlin.analysis.utils.collections.mapToSet
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.resolve.providers.FirSymbolNamesProvider
 import org.jetbrains.kotlin.fir.resolve.providers.FirDelegatingCachedSymbolNamesProvider
@@ -22,13 +23,13 @@ internal open class LLFirKotlinSymbolNamesProvider(
     override fun getTopLevelClassifierNamesInPackage(packageFqName: FqName): Set<String> =
         declarationProvider
             .getTopLevelKotlinClassLikeDeclarationNamesInPackage(packageFqName)
-            .mapTo(mutableSetOf()) { it.asString() }
+            .mapToSet { it.asString() }
 
     override fun getPackageNamesWithTopLevelCallables(): Set<String>? =
         declarationProvider.computePackageSetWithTopLevelCallableDeclarations()
 
     override fun getTopLevelCallableNamesInPackage(packageFqName: FqName): Set<Name> =
-        declarationProvider.getTopLevelCallableNamesInPackage(packageFqName)
+        declarationProvider.getTopLevelCallableNamesInPackage(packageFqName).ifEmpty { emptySet() }
 
     companion object {
         fun cached(session: FirSession, declarationProvider: KotlinDeclarationProvider): FirCachedSymbolNamesProvider =