`windows-rdl` gaps - header namespace orchestration

[kennykerr]

Gap 4 — Namespace manifest and orchestration tool
Why it matters: Win32 APIs span hundreds of SDK headers mapped to ~100 namespaces (Windows.Win32.Foundation, Windows.Win32.System.Com, Windows.Win32.UI.WindowsAndMessaging, etc.). The win32metadata project maintains a set of "partition" .h files that #include the relevant SDK headers for each namespace.

What to add:

A manifest format (YAML or TOML) that describes each namespace and the list of SDK headers (or a single umbrella header) it comes from, plus the library (DLL name) for functions, and any --filter suffixes to limit output to the target headers.
A new tool_win32 crate (or extend an existing tool) that reads the manifest, spawns one Clang::write call per namespace entry to produce per-namespace .rdl files, then drives Reader::write to compile each to a per-namespace .winmd, and finally calls tool_merge to combine all into Windows.Win32.winmd.
A previously compiled Windows.Win32.winmd (or the existing win32metadata file) can be passed as --reference to each Clang::write invocation so cross-namespace type references resolve correctly (which the existing ref_map mechanism already supports).

[kennykerr]

From #4314: The Windows API functions in the Windows SDK headers don't directly declare which DLL to use for importing the function. The Win32metadata project scrapes various libs and probably also has a bunch of manual fixups. We need a more reliable or at least more maintainable solution.

[kennykerr]

I suspect what we need is a windows-rdl repo with a master set of RDL files and a few tools that can update that master set. One tool will just run windows-rdl's Clang parser against the headers and update the master set of RDL files. Another tool will parse the onecoreuap.lib or whatever set of libs and update the RDL files accordingly. As the Windows SDK moves to eventually include winmd definitions for more APIs, we can shrink the dependence on the Clang parser and lib parser.

[riverar]

Will need to also add some flexibility to the lib parser as they are full of errors.