windows-rdl: clang parsing of constants

[Copilot]

Extends the windows-rdl clang module to evaluate complex #define constant expressions that the existing token-based parser cannot handle (arithmetic, bitwise shifts, cross-macro references, etc.) using the industry-standard synthetic-enum technique.

What's this all about?

The existing #define constant parser used token pattern-matching and only handled simple literal patterns and typed casts. Expressions like #define NAME OTHER + 123 or #define SHIFTED (FLAG << 16) were silently dropped.

This PR implements the industry-standard "synthetic enum" approach (also used by bindgen) as a second-pass fallback evaluator. For each macro that the token parser cannot handle, a dedicated anonymous enum is injected into an in-memory translation unit that #includes the original header. The C++ compiler evaluates the full constant expression — handling cross-macro references, arithmetic, bitshifts, and operator precedence — and the result is read via clang_getEnumConstantDeclValue. Using one enum per macro name combined with CXTranslationUnit_KeepGoing ensures that one bad macro (e.g. a string macro that isn't a valid integer constant expression) does not prevent the others from being evaluated.

Changes

• cx.rs: Added Index::parse_unsaved() for creating in-memory translation units from a CXUnsavedFile buffer using CXTranslationUnit_KeepGoing.
• const.rs: Added Const::evaluate_macros() — the synthetic-enum batch evaluator. Added F64 arm to write_const_value(). Validates macro names are safe C identifiers before interpolating into generated C++ source.
• mod.rs: Two-pass macro evaluation: first pass collects macros where parse_body returned None into a pending list; second pass calls Const::evaluate_macros() for those candidates.
• const.h / const.rdl: Added complex-expression test cases — cross-macro addition (FACILITY_DEBUGGER + 10 → 11) and bitshift (FACILITY_DEBUGGER << 16 → 65536).

Testing

• ✅ cargo fmt — clean
• ✅ cargo clippy — clean (removed unused EvalResult/Cursor::evaluate() dead code from an earlier approach)
• ✅ test_clang roundtrip test passes with the new complex-expression golden values
• ✅ enum.rdl golden restored to its original #[repr(i32)] — an unintended change introduced during development has been reverted

[Kenny Kerr (kennykerr)]

Copilot - be sure to cargo cargo fmt and cargo clippy.

[Copilot]

Copilot - be sure to cargo cargo fmt and cargo clippy.

Done in 401cc01. cargo fmt was clean; cargo clippy flagged the unused EvalResult struct and Cursor::evaluate() method (dead code from an earlier approach), so those have been removed.