SkSL: Rename isConstant to isCompileTimeConstant

This better reflects what the return value means, and eliminates
confusion with the SkSL concept of "const" variables, which are
not known at compile time.

Change-Id: I14973ceb36ae4ab6bad8a13bb93e1788a1b18bd9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/299863
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
14 files changed