generate include comments

- formalize how aliases and substitutions work together
- add constexpr, #define, typedef support
- check for correct description order
- write short enum, struct members

R=caryclark@google.com

Docs-Preview: https://skia.org/?cl=129455
Bug: skia:6898
Change-Id: Id60fc2ed02f38a7ba4e5cad5ef493d8c674e6183
Reviewed-on: https://skia-review.googlesource.com/129455
Commit-Queue: Cary Clark <caryclark@skia.org>
Reviewed-by: Cary Clark <caryclark@skia.org>
Auto-Submit: Cary Clark <caryclark@skia.org>
30 files changed