Reland "[vm,dart2bytecode,modular_aot] Variable-length closure objects"

This is a reland of commit 88496ba1c37fb6f67eac5920adb37eb8af0ee1f1

Fixes on top of the original change:

* Closure layout is changed to avoid gap between fixed fields and
  variable-length elements on compressed pointers architecture.
  This gap was causing crashes in the GC when scanning closure
  objects.

* pkg/vm_snapshot_analysis/test/instruction_sizes_test is fixed
  on arm64 by decreasing threshold for detecting size changes.

Original change's description:
> [vm,dart2bytecode,modular_aot] Variable-length closure objects
>
> Extend closure objects with variable number of elements to capture.
> This is needed to support capturing multiple independent contexts
> after capturing is computed in the front-end.
>
> The following fixed Closure fields are moved into variable-length
> elements:
>  - delayed type arguments;
>  - instantiator type arguments;
>  - function type arguments;
>  - context.
>
> Number of elements and presence/indices of various type arguments
> are encoded into the new length_and_flags field in the Closure.
>
> Most closure objects don't need any of the type arguments so this
> change will reduce average Closure object size.

TEST=ci
Issue: https://github.com/dart-lang/sdk/issues/61572
Issue: https://github.com/dart-lang/sdk/issues/61635

Change-Id: I8685e632e2d0832766ecdc470f3cf9a6b880de48
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/494243
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Tess Strickland <sstrickl@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>