Fixes eb7940b418.
As reported by Vladislav, there is possibly confusion on what exactly is
being ignored when there are multiple statements after DISCARD. Explicit
wrapping avoids that.
Differential Revision: https://code.wildfiregames.com/D3206
This was SVN commit r24397.
Introduce a DISCARD macro to ignore the warn_unused_result attribute
used by Spidermonkey, and reuse it elsewhere.
Differential Revision: https://code.wildfiregames.com/D3147
This was SVN commit r24261.