Wrap built-in symbol tables when an IRNode is cloned.

After an IRNode is cloned, callers expect to be able to safely mutate
its SymbolTable, so it can't be left with a built-in one.

Change-Id: If658fd11ad580da552f9d689edeeed4c842b38c9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/328384
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
6 files changed