add after_stack_push hook; add public ImmTy::from_immediate method, and make ImmTy::imm field private

move ModifiedStatic error to ConstEval errors, and generally adjust terminology from "static" to "global" where appropriate

This commit modifies the use of `subst_and_normalize_erasing_regions` on
parts of the MIR bodies returned from `instance_mir`, so that
`InstanceDef::CloneShim` and `InstanceDef::DropGlue` (where there is a
type) do not perform substitutions. This avoids double substitutions and
enables polymorphic `InstanceDef`s.
Signed-off-by: NDavid Wood <david@davidtw.co>

Also slightly refactor pointer bounds checks to avoid creating unnecessary temporary Errors

This reverts part of commit 9712fa40.

Co-Authored-By: NRalf Jung <post@ralfj.de>