Establish new test for shared discriminant

gcc/testsuite/ChangeLog:

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

Author: badumbatish

gcc/testsuite/rust/compile/shared_discriminator_enum.rs

@@ -0,0 +1,7 @@
+// from issue 3351: https://github.com/Rust-GCC/gccrs/issues/3351
+enum Foo { // { dg-error "discriminant value 1 assigned more than once" }
+    Bar = 1, 
+    Qaz = 1, 
+}
+
+fn main() {}