Add a regression test for unsafe module validation

Add a new test to check for unsafe modules during AST validation pass.

gcc/testsuite/ChangeLog:

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

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

gcc/testsuite/rust/compile/unsafe_module.rs

@@ -0,0 +1,2 @@
+unsafe mod toto {}
+// { dg-error "module cannot be declared unsafe" "" { target *-*-* } .-1 }