Bug 322271 – Windows Swift requires vcruntime modulemap override

• Component: WebKit Misc.
• Status: RESOLVED FIXED

To get our Swift code working on Windows, we need to override the limited vcruntime modulemap provided as part of the Windows Swift SDK to make it a bit more complete.

    1. The Swift SDK's vcruntime.modulemap wraps the MSVC STL in a clang
       module whose yvals_core.h lives in an explicit submodule, so the
       __cpp_lib_* feature-test macros never become visible to headers that
       reach <version> through a module import. WTF/simdutf then mix
       pre-C++20 and C++23 preprocessor paths. Seed the macros through the
       platform-swift-args.resp response file (generate-platform-args
       --stl-feature-macros); they must not go on the command line, which
       already sits near the 32767-char CreateProcess limit that ninja hits
       before swiftc-wrapper can expand its own response file.
    
    2. The same modulemap names only two of the STL's __msvc_*.hpp internal
       headers, leaving e.g. std::views::transform (__msvc_ranges_to.hpp)
       invisible at use sites. Generate an amended modulemap listing all of
       them and mount it over the MSVC include dir with a VFS overlay.
   

This problem has been reported upstream to the relevant Swift folks as rdar://185507163.

Pull request: https://github.com/WebKit/WebKit/pull/72124

[EWS]

Committed 319854@main (4e646d190c3f): <https://commits.webkit.org/319854@main>

Reviewed commits have been landed. Closing PR #72124 and removing active labels.

[Radar WebKit Bug Importer]

<rdar://problem/185874593>