test: add a regression test for issue #4145

Signed-off-by: Elle Rhumsaa <[email protected]>

Author: 0xllx0

gcc/testsuite/rust/compile/issue-4145.rs

@@ -0,0 +1,8 @@
+struct S {
+    field: [u8; {
+        #[path = "outer/inner.rs"]
+        mod inner;
+        // OK
+        0
+    }],
+}