swift234

[Type checker] Synthesize Error._code witnesses as 'final' within a class.

When synthesizing the witness for Error._code, synthesize it as
final. This isn't meant to be user-visible (and, therefore, isn't
meant to be user-overridable), so it's a minor efficiency
win. Moreover, we weren't making sure this member got synthesized in
in cross-module situations, leading to runtime crashes. Fixes
rdar://problem/27335637.