moved SkSL FunctionDefinition data into IRNode

Change-Id: Ia828de0793ee66301ba315f4593b4d7d69222b4e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/326717
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
18 files changed