gccrs: Add a test to highlight public trait type parsing

This new test highlight the parser's behavior around public trait types.

gcc/testsuite/ChangeLog:

	* rust/compile/trait_pub_type.rs: New test.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

gcc/testsuite/rust/compile/trait_pub_type.rs

@@ -0,0 +1,6 @@
+fn main() {}
+
+#[cfg(FALSE)]
+trait T {
+    pub type X;
+}