Mute "expect/actual classes are experimental" warning

This warning is introduced in Kotlin 1.9. KT-61573
If I don't mute this warning then the build fails because of `-Werror`

Author: nikitabobko

kotlinx-coroutines-core/build.gradle

@@ -170,6 +170,12 @@ kotlin {
     }
 }
 
+tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
+    kotlinOptions {
+        freeCompilerArgs += ['-Xexpect-actual-classes']
+    }
+}
+
 configurations {
     configureKotlinJvmPlatform(kotlinCompilerPluginClasspath)
 }

kotlinx-coroutines-test/build.gradle.kts

@@ -27,4 +27,8 @@ kotlin {
             }
         }
     }
+
+    compilerOptions {
+        freeCompilerArgs.add("-Xexpect-actual-classes")
+    }
 }