[vm] Implement `WeakReference` in the VM

This CL implements `WeakReference` in the VM.

* This reduces the size of weak references from 2 objects using 8 words
  to 1 object using 4 words.
* This makes loads of weak reference targets a single load instead of
  two.
* This avoids the fix-point in the GC and message object copying for
  weak references. (N.b. Weak references need to be processed _after_
  the fix-point for weak properties.)

The semantics of weak references in messages is that their target gets
set to `null` if the target is not included in the message by a strong
reference.

The tests take particular care to exercise the case where a weak
reference's target is only kept alive because a weak property key is
alive and it refers to the target in its value. This exercises the fact
that weak references need to be processed last.

Does not add support for weak references in the app snapshot. It would
be dead code until we start using weak references in for example the
CFE.

This CL does not try to unify weak references and weak properties in
the GC or messaging (as proposed in go/dart-vm-weakreference), because
their semantics differ enough.

Closes: https://github.com/dart-lang/sdk/issues/48162

TEST=runtime/tests/vm/dart/finalizer/weak_reference_run_gc_test.dart
TEST=runtime/tests/vm/dart/isolates/fast_object_copy_test.dart
TEST=runtime/vm/object_test.cc
TEST=tests/lib/isolate/weak_reference_message_1_test.dart
TEST=tests/lib/isolate/weak_reference_message_2_test.dart

Change-Id: I3810e919a5866f3ae8a95bd9aa23a880a0b0921c
Cq-Include-Trybots: luci.dart.try:app-kernel-linux-debug-x64-try,dart-sdk-mac-arm64-try,vm-canary-linux-debug-try,vm-fuchsia-release-x64-try,vm-kernel-gcc-linux-try,vm-kernel-asan-linux-release-x64-try,vm-kernel-linux-debug-x64c-try,vm-kernel-linux-debug-x64-try,vm-kernel-linux-debug-simriscv64-try,vm-kernel-mac-debug-x64-try,vm-kernel-nnbd-linux-debug-x64-try,vm-kernel-nnbd-linux-release-ia32-try,vm-kernel-nnbd-linux-release-simarm64-try,vm-kernel-nnbd-linux-release-simarm-try,vm-kernel-nnbd-mac-debug-arm64-try,vm-kernel-nnbd-mac-debug-x64-try,vm-kernel-nnbd-win-release-ia32-try,vm-kernel-nnbd-win-release-x64-try,vm-kernel-optcounter-threshold-linux-release-x64-try,vm-kernel-precomp-android-release-arm_x64-try,vm-kernel-precomp-linux-debug-x64-try,vm-kernel-precomp-linux-debug-x64c-try,vm-kernel-precomp-win-debug-x64c-try,vm-kernel-reload-rollback-linux-debug-x64-try,vm-kernel-reload-linux-debug-x64-try,vm-kernel-win-debug-ia32-try,vm-kernel-win-debug-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/232087
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>