Remove reflection

Out goes reflection! This means your code will break if you used the `:?` format specifier, anything else from libdebug, or the `visit_tydesc` intrinsic directly.

Closes #18046.

[breaking-change]
There's a bunch of code in glue.rs which is polymorphic over whether it is drop glue or visit glue. That could all be simplified now. It could be a follow-up though.