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/testData/diagnostics/testsWithStdLib/experimental/experimentalIsNotEnabled.kt

@@ -0,0 +1,23 @@
+// FILE: api.kt
+
+@<!EXPERIMENTAL_IS_NOT_ENABLED!>Experimental<!>
+annotation class Marker
+
+@Marker
+fun f() {}
+
+// FILE: usage.kt
+
+fun use1() {
+    <!EXPERIMENTAL_API_USAGE_ERROR!>f<!>()
+}
+
+@Marker
+fun use2() {
+    f()
+}
+
+@<!EXPERIMENTAL_IS_NOT_ENABLED!>UseExperimental<!>(Marker::class)
+fun use3() {
+    f()
+}