allow_internal_unstable is currently used
to side-step feature gate and stability checks.
While it was originally only meant to be used
only on macros, its use was expanded to
const functions.

This commit prepares stricter checks for the usage of allow_internal_unstable (only on macros)
and introduces the rustc_allow_const_fn_unstable attribute for usage on functions.

See rust-lang#69399

…able

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

Adds a check to make sure `#[rustc_allow_const_fn_unstable]`
can be applied only to functions.

Recognition for `rustc_allow_const_fn_unstable` attribute was errorneously
added in 05f4a9a.

…n rustc_mir

Followup rename from 05f4a9a,
which introduced `#[rustc_allow_const_fn_unstable]` for `const fn`s.

The validation was introduced in 3a63bf0
without strict validation of functions, e. g. all function types were
allowed.
Now the validation only allows `const fn`s.