[ty] Refactor SpecialFormType to use nested enums for type qualifiers and aliases #23517

Performance Gate Passed

✅ 24 untouched benchmarks
⏩ 30 skipped benchmarks¹

_{Comparing claude/test-specialformtype-structure-0KtJ5 (befefc3) with main (8dab7e4)}

30 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩