Require "-Xuse-experimental=kotlin.Experimental" on usages of Experimental

Since we're not yet sure of the design of Experimental/UseExperimental,
we're making them "experimental" themselves in some sense, in that the
user is required to provide the magic argument
"-Xuse-experimental=kotlin.Experimental" to be allowed to use either
Experimental or UseExperimental. This is more convenient than the
previous approach of "-language-version 1.3
-Xskip-metadata-version-check" because it's simpler and does not cause
pre-release binaries to be produced

compiler/tests/org/jetbrains/kotlin/codegen/JvmModuleProtoBufTest.kt

@@ -97,9 +97,7 @@ class JvmModuleProtoBufTest : KtUsefulTestCase() {
     fun testExperimental() {
         doTest(
             "/moduleProtoBuf/experimental", extraOptions = listOf(
-                "-Xskip-runtime-version-check",
-                "-language-version",
-                "1.3",
+                "-Xuse-experimental=kotlin.Experimental",
                 "-Xexperimental=org.foo.A",
                 "-Xexperimental=org.foo.B.C",
                 "-Xuse-experimental=org.foo.D"