validate allow_internal_unstable target

Adds a check to make sure `#[allow_internal_unstable]`
can be applied only to macro definitions.

src/test/ui/feature-gates/feature-gate-allow-internal-unstable-struct.rs

@@ -2,6 +2,7 @@
 // this needs a different test since this is done after expansion
 
 #[allow_internal_unstable()] //~ ERROR allow_internal_unstable side-steps
+//~| ERROR attribute should
 struct S;
 
 fn main() {}

src/test/ui/feature-gates/feature-gate-allow-internal-unstable-struct.stderr

@@ -6,6 +6,15 @@ LL | #[allow_internal_unstable()]
    |
    = help: add `#![feature(allow_internal_unstable)]` to the crate attributes to enable
 
-error: aborting due to previous error
+error: attribute should be applied to a macro
+  --> $DIR/feature-gate-allow-internal-unstable-struct.rs:4:1
+   |
+LL | #[allow_internal_unstable()]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |
+LL | struct S;
+   | --------- not a macro
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0658`.