Experimental checkers: simplify visiting code a bit

compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/ExperimentalMarkerDeclarationAnnotationChecker.kt

@@ -95,8 +95,7 @@ class ExperimentalMarkerDeclarationAnnotationChecker(private val module: ModuleD
         experimentalFqName: FqName,
         visited: MutableSet<CallableMemberDescriptor> = mutableSetOf()
     ): Boolean {
-        if (this in visited) return false
-        visited += this
+        if (!visited.add(this)) return false
         for (overridden in overriddenDescriptors) {
             if (overridden.annotations.any { it.fqName == experimentalFqName }) {
                 return true

compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/ExperimentalUsageChecker.kt

@@ -175,8 +175,7 @@ class ExperimentalUsageChecker(project: Project) : CallChecker {
             languageVersionSettings: LanguageVersionSettings,
             visited: MutableSet<DeclarationDescriptor> = mutableSetOf()
         ): Set<Experimentality> {
-            if (this in visited) return emptySet()
-            visited += this
+            if (!visited.add(this)) return emptySet()
             val result = SmartSet.create<Experimentality>()
             if (this is CallableMemberDescriptor && kind == CallableMemberDescriptor.Kind.FAKE_OVERRIDE) {
                 for (overridden in overriddenDescriptors) {