[SYCL] Replace direct builtins-side multi_ptr usage with forward-declaration

[koparasy]

sycl/builtins.hpp> no longer needs to include the full <sycl/multi_ptr.hpp> definition. Preserve multi_ptr builtin behavior once multi_ptr.hpp is included, and update include-deps / regression coverage.

On a TU including only <sycl/builtins.hpp> we observe 3.5% reduction for host compilation and 21.7% reduction for device compilation.

[koparasy]

PR Note (Breaking Change Awareness)

This change removes a transitive include of sycl/multi_ptr.hppfromsycl/builtins.hpp`.

As a result, code that previously relied on including sycl/builtins.hpp to implicitly obtain the definition of multi_ptr may no longer compile. Such usage was relying on an implementation detail rather than an explicit dependency.

Impact:

Source-breaking for applications that depend on transitive inclusion of multi_ptr.
No functional/runtime behavior changes.

Recommended fix for users:

Explicitly include sycl/multi_ptr.hpp where multi_ptr is used.
#include <sycl/multi_ptr.hpp>

Question for reviewers:

Should we:
(a) Treat this as an acceptable cleanup (users fix includes)
(b) Any suggestions?

[KornevNikita]

(a) Treat this as an acceptable cleanup (users fix includes)

Applications shouldn't use individual headers, but sycl.hpp. I don't think it's an issue.