Reduce memory size retained by LookupTrackerImpl

By default SmartList is used there for storing values of a multimap,
so a lot of duplicated items is stored there (file names are being
duplicated).

Even although there is a separate interner for these values,
retained memory size has been reduced for 30M when compiling `idea`
module in Kotlin project

build-common/src/org/jetbrains/kotlin/incremental/LookupStorage.kt

@@ -197,7 +197,7 @@ open class LookupStorage(private val targetDataDir: File) : BasicMapsOwner() {
 }
 
 class LookupTrackerImpl(private val delegate: LookupTracker) : LookupTracker {
-    val lookups = MultiMap<LookupSymbol, String>()
+    val lookups = MultiMap.createSet<LookupSymbol, String>()
     val pathInterner = StringInterner()
     private val interner = StringInterner()