Support Kotlin 1.5 / 1.6 language versions come from external sources

#KTIJ-15743 Fixed

Original commit: 784f77a2b98b0c9719f4919e45a193f35d7d9875

idea/idea-analysis/src/org/jetbrains/kotlin/idea/facet/KotlinVersionInfoProvider.kt

@@ -67,6 +67,8 @@ fun getDefaultLanguageLevel(
 
 fun String?.toLanguageVersion(): LanguageVersion = when {
     this == null -> VersionView.RELEASED_VERSION
+    startsWith("1.6") -> LanguageVersion.KOTLIN_1_6
+    startsWith("1.5") -> LanguageVersion.KOTLIN_1_5
     startsWith("1.4") -> LanguageVersion.KOTLIN_1_4
     startsWith("1.3") -> LanguageVersion.KOTLIN_1_3
     startsWith("1.2") -> LanguageVersion.KOTLIN_1_2

idea/tests/org/jetbrains/kotlin/idea/facet/KotlinVersionInfoProviderTest.kt

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2010-2021 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.idea.facet
+
+import org.jetbrains.kotlin.config.LanguageVersion
+import org.junit.Test
+import kotlin.test.assertSame
+
+class KotlinVersionInfoProviderTest {
+    @Test
+    fun supportedLanguageVersionConsistency() {
+        LanguageVersion.values().forEach { languageVersion ->
+            assertSame(languageVersion, languageVersion.toString().toLanguageVersion())
+        }
+    }
+}