Reland "[utils] Adds Span::FillBytes as a workaround for ASAN timeouts."

- Fixes the compilation error by wrapping the memset call with
  DAWN_UNSAFE_BUFFER and corresponding SAFETY message.

This is a reland of commit d0dc9fd134a521b7e68605a7bc6769c273e3e51b

Bug: 545842615

Original change's description:
> [utils] Adds Span::FillBytes as a workaround for ASAN timeouts.
>
> - In the bug below, we identified that as a part of spanification,
>   we updated some memset calls to std::ranges::fill. While this
>   was good for normal builds, this caused timeout issues in ASAN
>   builds because under ASAN std::ranges::fill obscures the loop
>   structure from the compiler's loop recognition. When dealing
>   with large buffers, this translates to billions of instrumented
>   stores causing timeouts.
> - This change introduces the Span::FillBytes helper that addresses
>   these timeouts by falling back to using memset when built for
>   ASAN.
>
> Bug: 545842615
> Change-Id: I431d16af99036cf8a2ba45ef173aa119e115804f
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/338335
> Commit-Queue: dan sinclair <dsinclair@chromium.org>
> Auto-Submit: Loko Kung <lokokung@google.com>
> Reviewed-by: dan sinclair <dsinclair@chromium.org>
> Reviewed-by: Antonio Maiorano <amaiorano@google.com>

Bug: 545842615
Change-Id: Idba5200432ffb9e8a04918ba947a1b383ce6edc8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/338795
Commit-Queue: Loko Kung <lokokung@google.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Auto-Submit: Loko Kung <lokokung@google.com>