Turn ObjcSelector into a struct to avoid empty vtable bug

[yebblies]

@WalterBright

DMD has a bug where it emits an empty vtable for extern(C++) classes that don't have any virtual methods, causing a mismatch and crashes etc if it's ever passed across the boundary. The only classes like this currently in dmd are Lexer/Parser, which never cross the boundary.

[WalterBright]

Doesn't pulling this require a re-do of #4884 ? This should be fixed after pulling #4884 , which should be the top priority.

[dnadlinger]

#4884 currently does not pass the auto tester on OS X. Martin has also added a commit addressing this.

[WalterBright]

I see:
 auto-tester — Pass: 10
for #4884

[yebblies]

This is a prerequisite for the switch to DDMD. There are about 3 more of these. Once you pull this I will do the next one.

[WalterBright]

I don't know why, but I'll take your word for it :-)

[yebblies]

Thanks