Implement new assert semantics in back-end

Previously, assert was just a regular function and its argument used to
be computed on each call (even if assertions are disabled on JVM).
This change adds support for 3 new behaviours of assert:
* always-enable (independently from -ea on JVM)
* always-disable (independently from -ea JVM)
* runtime/jvm (compile the calls like javac generates assert-operator)
* legacy (leave current eager semantics) - this already existed

Default behaviour is legacy for now.

The behavior is changed based on -Xassertions flag.
 #KT-7540: Fixed

compiler/frontend.java/src/org/jetbrains/kotlin/config/JVMAssertionsMode.kt

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. 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.config
+
+enum class JVMAssertionsMode(val description: String) {
+    ALWAYS_ENABLE("always-enable"),
+    ALWAYS_DISABLE("always-disable"),
+    JVM("jvm"),
+    LEGACY("legacy");
+
+    companion object {
+        @JvmField
+        val DEFAULT = LEGACY
+
+        @JvmStatic
+        fun fromStringOrNull(string: String?) = values().find { it.description == string }
+    }
+}

compiler/frontend.java/src/org/jetbrains/kotlin/config/JVMConfigurationKeys.java

@@ -65,6 +65,8 @@ public class JVMConfigurationKeys {
             CompilerConfigurationKey.create("disable not-null call receiver assertions");
     public static final CompilerConfigurationKey<Boolean> DISABLE_PARAM_ASSERTIONS =
             CompilerConfigurationKey.create("disable not-null parameter assertions");
+    public static final CompilerConfigurationKey<JVMAssertionsMode> ASSERTIONS_MODE =
+            CompilerConfigurationKey.create("assertions mode");
     public static final CompilerConfigurationKey<JVMConstructorCallNormalizationMode> CONSTRUCTOR_CALL_NORMALIZATION_MODE =
             CompilerConfigurationKey.create("constructor call normalization mode");
     public static final CompilerConfigurationKey<Boolean> NO_EXCEPTION_ON_EXPLICIT_EQUALS_FOR_BOXED_NULL =