add rustc_allow_const_fn_unstable attribute

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/rust#69399

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