Add tests

tests/ui/attributes/unsafe/cfg-unsafe-attributes.rs

@@ -0,0 +1,7 @@
+//@ build-pass
+#![feature(unsafe_attributes)]
+
+#[cfg_attr(all(), unsafe(no_mangle))]
+fn a() {}
+
+fn main() {}

tests/ui/attributes/unsafe/unsafe-attributes.rs

@@ -0,0 +1,7 @@
+//@ build-pass
+#![feature(unsafe_attributes)]
+
+#[unsafe(no_mangle)]
+fn a() {}
+
+fn main() {}