FIR: add EXPERIMENTAL_API_USAGE reporting

compiler/testData/diagnostics/testsWithStdLib/experimental/experimentalSetter.kt

@@ -0,0 +1,24 @@
+// FIR_IDENTICAL
+// !USE_EXPERIMENTAL: kotlin.RequiresOptIn
+
+@RequiresOptIn
+@Retention(AnnotationRetention.BINARY)
+annotation class E
+
+@set:E
+var x: Int = 42
+
+@E
+var y: Int = 24
+
+var z: Int = 44
+    @E set(arg) {
+        field = arg
+    }
+
+fun user(): Int {
+    <!EXPERIMENTAL_API_USAGE_ERROR!>x<!> = 10
+    <!EXPERIMENTAL_API_USAGE_ERROR!>y<!> = 5
+    <!EXPERIMENTAL_API_USAGE_ERROR!>x<!> = 15
+    return x + <!EXPERIMENTAL_API_USAGE_ERROR!>y<!> + z
+}