swift234

Perform the class-or-ObjC-existential checks first in the runtime bridging entrypoints.

We decided that classes and Objective-C existentials are always going
to be bridged verbatim, ignoring any _BridgedToObjectiveC conformances.
While the compiler isn't preventing conformance of a class type to
_BridgedToObjectiveC, we ignore this case and will find a way to lock
it down if _BridgedToObjectiveC loses its underscore.

The motivation here is that (1) we don't really have a use case for a
class that bridges to Objective-C in any way other than just being an
Objective-C class, and (2) the class-or-ObjC-existential check is
ridiculously cheap compared to the
thread-safe-hash-table-over-a-dlsym-call used to find witness tables,
so do the cheap, common check first.



Swift SVN r18177