[SYCL] reapproaching guard duty

[cperkinsintel]

Our fix for VS2026 was being guarded by sycl_global_var so as to not trigger "non const global in device code" warnings. But that doesn't work when preprocessing ( -E flag), so now we are adjusting the guard to be closer to where the diagnostic is actually generated. This is is more straightforward, less magical, and exactly the same mechanism that is already used (isMsvcMathFn).

[Copilot]

Pull request overview

Note

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Adjusts the MSVC STL global-variable handling for SYCL device code so diagnostics are suppressed at the point they’re emitted (Sema use-sites), and removes the fragile system_header/sycl_global_var workaround that breaks under -E preprocess round-trips.

Changes:

• Removes #pragma clang system_header and sycl_global_var usage from the MSVC bit utils wrapper.
• Adds a Sema allowlist to permit reads of specific MSVC STL runtime globals (e.g. std::__isa_available) from device code.
• Adds a regression test covering -E preprocess → .ii → recompile round-trip.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
sycl/test/regression/msvc_bit_utils_wrapper_preprocess_roundtrip.cpp	Adds a preprocess round-trip regression test for the wrapper/header scenario.
sycl/include/sycl/stl_wrappers/__msvc_bit_utils.hpp	Removes the system-header + attribute workaround and relies on Sema allowlisting.
clang/lib/Sema/SemaExpr.cpp	Introduces std::__isa_available read allowlisting for Windows MSVC environments.