Clear the `builtin` status of SkSL nodes when they are cloned.

This has a slight ripple effect into Enum, as it was using the builtin
status as an indicator that the enum was shared with C++ code. This now
has a dedicated bool flag.

Change-Id: Id03efa902546775666acd031e6d57123e02b6c6e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/328381
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
6 files changed