Improve message for raw pointer missing mut and const

"Bare raw pointer" does not exist as a concept.

src/test/parse-fail/bad-pointer-type.rs

@@ -11,5 +11,5 @@
 // compile-flags: -Z parse-only
 
 fn foo(_: *()) {
-    //~^ bare raw pointers are not allowed, use `*mut T` or `*const T` as appropriate
+    //~^ expected mut or const in raw pointer type (use `*mut T` or `*const T` as appropriate)
 }