Migrate SkSLExternalFunction to the ir/ directory.

Change-Id: I8ca4fbba551624c61a4ebe7a4716750fe0b48196
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/395818
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
6 files changed