Support "// !API_VERSION: LATEST" directive in tests

Useful for tests that check behavior which depends on having a recent
API version enabled, and that don't want to depend on a specific fixed
API version (to prevent eventual obsoletion)

compiler/util/src/org/jetbrains/kotlin/config/ApiVersion.kt

@@ -52,6 +52,9 @@ class ApiVersion private constructor(
         @JvmField
         val KOTLIN_1_3 = createByLanguageVersion(LanguageVersion.KOTLIN_1_3)
 
+        @JvmField
+        val LATEST: ApiVersion = createByLanguageVersion(LanguageVersion.values().last())
+
         @JvmField
         val LATEST_STABLE: ApiVersion = createByLanguageVersion(LanguageVersion.LATEST_STABLE)