Add -Xopt-in command line argument

As per https://github.com/Kotlin/KEEP/pull/201

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

@@ -175,6 +175,13 @@ abstract class CommonCompilerArguments : CommonToolArguments() {
     )
     var useExperimental: Array<String>? by FreezableVar(null)
 
+    @Argument(
+        value = "-Xopt-in",
+        valueDescription = "<fq.name>",
+        description = "Enable usages of API that requires opt-in with an opt-in requirement marker with the given fully qualified name"
+    )
+    var optIn: Array<String>? by FreezableVar(null)
+
     @Argument(
         value = "-Xproper-ieee754-comparisons",
         description = "Generate proper IEEE 754 comparisons in all cases if values are statically known to be of primitive numeric types"
@@ -333,7 +340,7 @@ abstract class CommonCompilerArguments : CommonToolArguments() {
                 put(AnalysisFlags.experimental, experimentalFqNames)
                 collector.report(CompilerMessageSeverity.WARNING, "'-Xexperimental' is deprecated and will be removed in a future release")
             }
-            put(AnalysisFlags.useExperimental, useExperimental?.toList().orEmpty())
+            put(AnalysisFlags.useExperimental, useExperimental?.toList().orEmpty() + optIn?.toList().orEmpty())
             put(AnalysisFlags.explicitApiVersion, apiVersion != null)
             put(AnalysisFlags.allowResultReturnType, allowResultReturnType)
             ExplicitApiMode.fromString(explicitApi)?.also { put(AnalysisFlags.explicitApiMode, it) } ?: collector.report(