Triple: Add BridgeOS to isOSDarwin (#145636)
This fixes a TODO and avoids a special case. Also required
hacking up a few cases to avoid asserting in codegen; it's not
confidence inspiring that there is only one codegen test using
a bridgeos triple and its specifically for the exp10 libcall
names.
This also changes the behavior, losing an extra leading _ in the
emitted name matching the other apple outputs. I have no idea if
this is right or not. IMO it's someone from apple's problem to fix
it and add appropriate test coverage, or we can rip all references
to BridgeOS out from upstream.
