Refactored memset and memcpy calls to D slices in multiple files

[AbhayPratap05]

This PR replaces C-style memcpy and memset calls with D slice operations in several files:

• compiler/src/dmd/common/outbuffer.d
• compiler/src/dmd/dcast.d
• compiler/src/dmd/root/array.d
• compiler/src/dmd/root/bitarray.d

Changes made:

1. Replaced memset calls with D slice assignments:
   (cast(ubyte*)ptr)[0 .. size] = 0
2. Replaced memcpy calls with D slice copying:
   (cast(ubyte*)dest)[0 .. size] = (cast(const(ubyte)*)src)[0 .. size]

Testing:

• Successfully built the compiler using Docker
• Ran tests in the Docker container

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#21147"

[ibuclaw]

Hi, thanks for the PR.

What is the net benefit of this change?

From a code-gen stance, both current and proposed changes are identical.

The current code is arguably more readable too. Drop the casts, and it would start to look like an improvement.

[dkorpel]

Thanks for the changes to dcast.d.

OutBuffer and Array are very common optimized abstractions based on low-level building blocks. As a consequence, many of the translated slice expressions are more complex than the original memset/memcpy. On top of that, since slices have bounds checking, there is a performance regression risk (since we don't have the tester from the GSoC project yet), so please refrain from those refactorings for now.

[AbhayPratap05]

This PR reverts the previous slice-based casts and instead:

• Reverts all memset/memcpy replacements in outbuffer.d, array.d and bitarray.d
• Updates dcast.d to use casting at allocation site instead of casting in slice operations

The changes follows mentor feedback to avoid performance regressions in OutBuffer and Array.