Treat uninvoked external-function-references as errors.

Finding a dangling ExternalFunctionReference in the IR should be handled
the same as a FunctionReference; both are equally indicative of a
problem in the user's code.

Change-Id: I5aa9204e692678d7b54d78fc7253d49940dd130a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/442694
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>
2 files changed