gccrs: Add test for testsuite

Adds simple test with unused Enum.

Addresses #4367

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4367.rs: New test.

Signed-off-by: lenny.chiadmi-delage <lenny.chiadmi-delage@epita.fr>

Author: Villosse

gcc/testsuite/rust/compile/issue-4367.rs

@@ -0,0 +1,10 @@
+enum Foo {
+    // { dg-warning "enum is never used" "" { target *-*-* } .-1 }
+    A,
+    B,
+}
+
+enum _Bar {
+    A,
+    B,
+}