Improved DSL Struct error reporting

This adds position tracking to individual fields and an error for
illegal field modifiers.

Change-Id: Ie121699bde94e831f33a0bc021349a1cd757d08b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/443888
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
2 files changed