Fix thread-unsafe writes to function declarations in the symbol table.

This changes the semantics *slightly* as `fIsBuiltinCode` is used
to identify functions which are assumed to not have side effects.
Previously the heuristic for side-effects was the presence of a function
body. In practical terms this does not seem to affect any results. (I
also tried other approaches which all subtly altered the results of
codegen/inlining.)

Change-Id: I9fea820968a60360fb622dfa70aa8876a1782e6a
Bug: skia:10589
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/312336
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
3 files changed