Add parser support for highp/mediump Tokens in vardecls.

These parse into new modifier bits; the IR generator does not yet
support these bits. That's coming in a followup CL.

Change-Id: I362e9227694f9b862eaad100f6afca45a9b62a01
Bug: skia:12248
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436336
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
12 files changed