[FE 1.0] Make "Unit conversions" accessible experimental feature

compiler/testData/diagnostics/tests/unitConversion/kt49394.kt

@@ -0,0 +1,15 @@
+fun interface Run {
+    fun run()
+}
+
+fun handle(run: Run) {
+    //...
+}
+
+val x = {
+    "STRING"
+}
+
+fun test() {
+    handle(<!UNSUPPORTED_FEATURE("The feature "unit conversions on arbitrary expressions" is experimental and should be enabled explicitly")!>x<!>)
+}