Add new test for invalid variadics

Highlight invalid variadic filtering through the ast validation checker.

gcc/testsuite/ChangeLog:

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

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

gcc/testsuite/rust/compile/invalid_variadics.rs

@@ -0,0 +1,6 @@
+extern "C" {
+    pub fn dog(b: i32, a: ..., c: i32);
+    // { dg-error "..... must be the last argument of a C-variadic function" "" { target *-*-* } .-1 }
+    pub fn cat(a: ...);
+    // { dg-error "C-variadic function must be declared with at least one named argument" "" { target *-*-* } .-1 }
+}