gccrs: add test case to show issue is fixed

Fixes #3599

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3599.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

Author: philberty

gcc/testsuite/rust/compile/issue-3599.rs

@@ -0,0 +1,8 @@
+#[lang = "sized"]
+trait Sized {}
+
+trait Bar {}
+
+struct S; // { dg-warning "struct is never constructed" }
+
+pub fn test(foo: impl Bar) {}