better

Fixes #29857.
Fixes #30589.

Previously it was returning a value, mostly for the two reasons:

* Cloning Lvalue is very cheap most of the time (i.e. when Lvalue is not a Projection);
* There’s users who want &mut lvalue and there’s users who want &lvalue. Returning a value allows
  to make either one easier when pattern matching (i.e. Some(ref dest) or Some(ref mut dest)).

However, I’m now convinced this is an invalid approach. Namely the users which want a mutable
reference may modify the Lvalue in-place, but the changes won’t be reflected in the final MIR,
since the Lvalue modified is merely a clone.

Instead, we have two accessors `destination` and `destination_mut` which return a reference to the
destination in desired mode.

Fixes #28953

Create a visitor for `TypeFoldable`s and use it to implement RegionEscape and HasTypeFlags (fixes #20298)

This merges two separate Call terminators and uses a separate CallKind sub-enum instead.

A little bit unrelatedly, copying into destination value for a certain kind of invoke, is also
implemented here. See the associated comment in code for various details that arise with this
implementation.

It is useful for various cases where direct unreachable cannot be translated and a separate block
is necessary.

Unreachable terminator can be contained all within the trans.

Diverge should eventually go away

DivergingCall is different enough from the regular converging Call to warrant the split. This also
inlines CallData struct and creates a new CallTargets enum in order to have a way to differentiate
between calls that do not have an associated cleanup block.

Note, that this patch still does not produce DivergingCall terminator anywhere. Look for that in
the next patches.

we were using interior mutability (RefCells, TyIvar), add some reads/writes.

along with a README explaining how they are to be used