FIXME: de-uglify

No functional changes intended.

This matters if the lvalues that is suspended involves Deref'ing a reference --
that reference's lifetime will then not be in the type any more

add new instructions for asserting when values are valid, and to describe when we can rely on them being locked in memory

Leaving types unerased would lead to 2 types with a different "name"
getting different move-paths, which would cause major brokenness (see
e.g. #42903).

This does not fix any *known* issue, but is required if we want to use
abs_domain with non-erased regions (because the same can easily
have different names). cc @RalfJung.

 * Emit `EndRegion` for every code-extent for which we observe a
   borrow. To do this, we needed to thread source info back through
   to `fn in_scope`, which makes this commit a bit more painful than
   one might have expected.

 * There is `end_region` emission in `Builder::pop_scope` and in
   `Builder::exit_scope`; the first handles falling out of a scope
   normally, the second handles e.g. `break`.

 * Remove `EndRegion` statements during the erase_regions mir
   transformation.

 * Preallocate the terminator block, and throw an `Unreachable` marker
   on it from the outset. Then overwrite that Terminator as necessary
   on demand.

 * Instead of marking the scope as needs_cleanup after seeing a
   borrow, just treat every scope in the chain as being part of the
   diverge_block (after any *one* of them has separately signalled
   that it needs cleanup, e.g. due to having a destructor to run).

 * Allow for resume terminators to be patched when looking up drop flags.

   (In particular, `MirPatch::new` has an explicit code path,
   presumably previously unreachable, that patches up such resume
   terminators.)

 * Make `Scope` implement `Debug` trait.

 * Expanded a stray comment: we do not emit StorageDead on diverging
   paths, but that end behavior might not be desirable.

This initial commit provides implementations for HIR, MIR, and
everything that also needs to be supported for those two.