The rustc_allowed_through_unstable_modules attribute lets users call std::mem::transmute as std::intrinsics::transmute. The former is a reexport of the latter, and for a long time we didn't properly check stability for reexports, so making this a hard error now would be a breaking change for little gain. But at the same time, std::intrinsics::transmute is not the intended path for this function, so I think it is a good idea to show a deprecation warning when that path is used. This PR implements that, for all the functions in std::intrinsics that carry the attribute.

I assume this will need @rust-lang/libs-api FCP.