Moved findAndDeclareBuiltinVariables out of IRGenerator

No functional changes, just continuing the process of moving code out
of IRGenerator in hopes of its eventual disappearance.

Change-Id: I3507b85c5493c61812869c9bae53de8af89ee129
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/453941
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
6 files changed