JVM IR: report warning instead of error for language/API version 1.2

This is needed for testing JVM IR on existing projects which for some
reason are still on Kotlin 1.2.

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JVMCompilerArguments.kt

@@ -360,8 +360,9 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
             || languageVersionSettings.apiVersion < ApiVersion.KOTLIN_1_3
         ) {
             collector.report(
-                CompilerMessageSeverity.ERROR,
+                CompilerMessageSeverity.STRONG_WARNING,
-                "IR backend cannot be used with language or API version below 1.3"
+                "IR backend does not support language or API version lower than 1.3. " +
+                        "This can lead to unexpected behavior or compilation failures"
             )
         }
     }

compiler/testData/cli/jvm/irApiVersionUnsupported.out

@@ -1,3 +1,3 @@
 warning: API version 1.2 is deprecated and its support will be removed in a future version of Kotlin
-error: IR backend cannot be used with language or API version below 1.3
+warning: IR backend does not support language or API version lower than 1.3. This can lead to unexpected behavior or compilation failures
-COMPILATION_ERROR
+OK

compiler/testData/cli/jvm/irLanguageVersionUnsupported.out

@@ -1,3 +1,3 @@
 warning: language version 1.2 is deprecated and its support will be removed in a future version of Kotlin
-error: IR backend cannot be used with language or API version below 1.3
+warning: IR backend does not support language or API version lower than 1.3. This can lead to unexpected behavior or compilation failures
-COMPILATION_ERROR
+OK