Fix -Xmulti-platform dropped from args when freeCompilerArgs is assigned

in user build script

Issue #KT-15371 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/BaseGradleIT.kt

@@ -465,6 +465,9 @@ abstract class BaseGradleIT {
     private fun Project.createBuildCommand(wrapperDir: File, params: Array<out String>, options: BuildOptions): List<String> =
             createGradleCommand(wrapperDir, createGradleTailParameters(options, params))
 
+    fun Project.gradleBuildScript(subproject: String? = null): File =
+        File(projectDir, subproject?.plus("/").orEmpty() + "build.gradle")
+
     private fun Project.createGradleTailParameters(options: BuildOptions, params: Array<out String> = arrayOf()): List<String> =
             params.toMutableList().apply {
                 add("--stacktrace")

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/MultiplatformGradleIT.kt

@@ -185,4 +185,20 @@ class MultiplatformGradleIT : BaseGradleIT() {
             }
         }
     }
+
+    @Test
+    fun testFreeCompilerArgsAssignment(): Unit = with(Project("multiplatformProject")) {
+        setupWorkingDir()
+
+        val overrideCompilerArgs = "kotlinOptions.freeCompilerArgs = ['-verbose']"
+
+        gradleBuildScript("lib").appendText("\ncompileKotlinCommon.$overrideCompilerArgs")
+        gradleBuildScript("libJvm").appendText("\ncompileKotlin.$overrideCompilerArgs")
+        gradleBuildScript("libJs").appendText("\ncompileKotlin2Js.$overrideCompilerArgs")
+
+        build("build") {
+            assertSuccessful()
+            assertTasksExecuted(listOf(":lib:compileKotlinCommon", ":libJvm:compileKotlin", ":libJs:compileKotlin2Js"))
+        }
+    }
 }