This fixes the edge case we had with variables pointing to dynamic
blocks, which forced us to convert basically *all* dynamic blocks to
DeadBlock when deallocating them.
We now don't run dynamic blocks through InterpState::deallocate() but
instead add them to a DeadAllocations list when they are deallocated but
still have pointers.
As a consequence, not all blocks with Block::IsDead = true are
DeadBlocks.
First, the old MoveFn was rather inefficient, since the dead data cannot
ever be accessed anyway.
Second, there was a problem where the only reason a block still had a
pointer to it (and thus was made into a DeadBlock instead of simply
being deallocated) as that a nested field in the block pointed to the
block itself.
Fix this by calling the dtor function unconditionally. If the block
*still* has pointers after that, we really need to create a DeadBlock
for it.
Instead of heap-allocating it. This is similar to what the current
interpeter does. In C, we have no function calls, so the extra heap
allocation never makes sense.
And fix the diagnostics for __builtin_is_constant_evaluated(). We can be
in a non-constant context, but calling an immediate function always
makes the context constant for the duration of that call.
