Add regression test for unsafe mods

Author: dtolnay

Diff for: tests/source/unsafe-mod.rs

@@ -0,0 +1,7 @@
+// These are supported by rustc syntactically but not semantically.
+
+#[cfg(any())]
+unsafe mod m { }
+
+#[cfg(any())]
+unsafe extern "C++" { }

Diff for: tests/target/unsafe-mod.rs

@@ -0,0 +1,7 @@
+// These are supported by rustc syntactically but not semantically.
+
+#[cfg(any())]
+unsafe mod m {}
+
+#[cfg(any())]
+unsafe extern "C++" {}