Get @constructor feature of SKSL working

This is probably the only portion of https://skia-review.googlesource.com/c/skia/+/115900 that we want to keep

TBR=ethannicholas@google.com
Change-Id: Ife4d7bfbe454f705e93fb051b0bcf8e2b0a052b5
Reviewed-on: https://skia-review.googlesource.com/116192
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed