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

libraries/tools/binary-compatibility-validator/src/test/kotlin/org.jetbrains.kotlin.tools.tests/RuntimePublicAPITest.kt

@@ -35,7 +35,7 @@ class RuntimePublicAPITest {
     @Test fun kotlinStdlibRuntimeMerged() {
         snapshotAPIAndCompare(
             "../../stdlib/jvm/build/libs", "kotlin-stdlib",
-            listOf("../stdlib-declarations.json", "../stdlib-experimental-declarations.json"),
+            listOf("../stdlib-declarations.json"/*, "../stdlib-experimental-declarations.json"*/),
             listOf("kotlin.jvm.internal")
         )
     }