Warn when running the compiler under Java 6 or 7

(cherry picked from commit 5537800)

(cherry picked from commit 5614874)

compiler/cli/src/org/jetbrains/kotlin/cli/common/CLICompiler.java

@@ -155,6 +155,7 @@ public abstract class CLICompiler<A extends CommonCompilerArguments> {
         }
 
         reportUnknownExtraFlags(messageCollector, arguments);
+        reportUnsupportedJavaVersion(messageCollector, arguments);
 
         GroupingMessageCollector groupingCollector = new GroupingMessageCollector(messageCollector);
 
@@ -350,6 +351,16 @@ public abstract class CLICompiler<A extends CommonCompilerArguments> {
         }
     }
 
+    private void reportUnsupportedJavaVersion(MessageCollector collector, A arguments) {
+        if (!SystemInfo.isJavaVersionAtLeast("1.8") && !arguments.noJavaVersionWarning) {
+            collector.report(
+                    CompilerMessageSeverity.STRONG_WARNING,
+                    "Running the Kotlin compiler under Java 6 or 7 is unsupported and will no longer be possible in a future update.",
+                    CompilerMessageLocation.NO_LOCATION
+            );
+        }
+    }
+
     @NotNull
     protected abstract ExitCode doExecute(
             @NotNull A arguments,