Add a non-public-API mechanism to import free args for MPP source sets

Arguments can be set using a project extra property following the
pattern: `kotlin.mpp.freeCompilerArgsForSourceSet.$sourceSetName`.

Only a compilation's default source set free args are appended to the
compiler args during Gradle builds.

idea/idea-gradle/src/org/jetbrains/kotlin/idea/configuration/KotlinMPPGradleProjectResolver.kt

@@ -891,6 +891,7 @@ open class KotlinMPPGradleProjectResolver : AbstractProjectResolverExtension() {
                     it.useExperimental = languageSettings.experimentalAnnotationsInUse.toTypedArray()
                     it.pluginOptions = languageSettings.compilerPluginArguments
                     it.pluginClasspaths = languageSettings.compilerPluginClasspath.map(File::getPath).toTypedArray()
+                    it.freeArgs = languageSettings.freeCompilerArgs.toMutableList()
                 }
             }
         }

idea/kotlin-gradle-tooling/src/KotlinMPPGradleModel.kt

@@ -79,6 +79,7 @@ interface KotlinLanguageSettings : Serializable {
     val experimentalAnnotationsInUse: Set<String>
     val compilerPluginArguments: Array<String>
     val compilerPluginClasspath: Set<File>
+    val freeCompilerArgs: Array<String>
 }
 
 interface KotlinCompilationOutput : Serializable {

idea/kotlin-gradle-tooling/src/KotlinMPPGradleModelBuilder.kt

@@ -214,6 +214,7 @@ class KotlinMPPGradleModelBuilder : ModelBuilderService {
         val getExperimentalAnnotationsInUse = languageSettingsClass.getMethodOrNull("getExperimentalAnnotationsInUse")
         val getCompilerPluginArguments = languageSettingsClass.getMethodOrNull("getCompilerPluginArguments")
         val getCompilerPluginClasspath = languageSettingsClass.getMethodOrNull("getCompilerPluginClasspath")
+        val getFreeCompilerArgs = languageSettingsClass.getMethodOrNull("getFreeCompilerArgs")
         @Suppress("UNCHECKED_CAST")
         return KotlinLanguageSettingsImpl(
             getLanguageVersion(gradleLanguageSettings) as? String,
@@ -222,7 +223,8 @@ class KotlinMPPGradleModelBuilder : ModelBuilderService {
             getEnabledLanguageFeatures(gradleLanguageSettings) as? Set<String> ?: emptySet(),
             getExperimentalAnnotationsInUse?.invoke(gradleLanguageSettings) as? Set<String> ?: emptySet(),
             (getCompilerPluginArguments?.invoke(gradleLanguageSettings) as? List<String> ?: emptyList()).toTypedArray(),
-            (getCompilerPluginClasspath?.invoke(gradleLanguageSettings) as? FileCollection)?.files ?: emptySet()
+            (getCompilerPluginClasspath?.invoke(gradleLanguageSettings) as? FileCollection)?.files ?: emptySet(),
+            (getFreeCompilerArgs?.invoke(gradleLanguageSettings) as? List<String>).orEmpty().toTypedArray()
         )
     }
 

idea/kotlin-gradle-tooling/src/KotlinMPPGradleModelImpl.kt

@@ -66,7 +66,8 @@ data class KotlinLanguageSettingsImpl(
     override val enabledLanguageFeatures: Set<String>,
     override val experimentalAnnotationsInUse: Set<String>,
     override val compilerPluginArguments: Array<String>,
-    override val compilerPluginClasspath: Set<File>
+    override val compilerPluginClasspath: Set<File>,
+    override val freeCompilerArgs: Array<String>
 ) : KotlinLanguageSettings {
     constructor(settings: KotlinLanguageSettings) : this(
         settings.languageVersion,
@@ -75,7 +76,8 @@ data class KotlinLanguageSettingsImpl(
         settings.enabledLanguageFeatures,
         settings.experimentalAnnotationsInUse,
         settings.compilerPluginArguments,
-        settings.compilerPluginClasspath
+        settings.compilerPluginClasspath,
+        settings.freeCompilerArgs
     )
 }