Miri: replace canonical_alloc_id mechanism by extern_static_alloc_id which is called only when a pointer is 'imported' into the machine

Add function eval_maybe_thread_local_static_const that allows handling thread locals without touching debug info; address other PR comments.

asm! is left as a wrapper around llvm_asm! to maintain compatibility.

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>

In Cargo.toml, the opt-level for `release` and `bench` was
overridden to be 2. This was to work around a problem with LLVM
7. However, rust no longer uses LLVM 7, so this is no longer
needed.

This creates a small compile time regression in MIR constant eval,
so I've added a #[inline(always)] on the `step` function used in
const eval

Also creates a binary size increase in wasm-stringify-ints-small,
so I've bumped the limit there.

This operator creates a raw pointer to a Place directly, without first
creating a reference. See RFC #2582 for motivation.

The Rvalue is currently unused.

This commits adds in some additional hooks to allow Miri to properly
handle panic unwinding. None of this should have any impact on CTFE mode

fix an ICE

fix method name