forcing one to declare, e.g., [infix_left=NNN] on every
declaration. The precedence/associativity will be merged from any
similarly-named operator in translation unit scope or in an imported
module. Fixes <rdar://problem/11261874> very, very narrowly for the
demo.
This implementation is hacky and slow; we'll replace it later with
<rdar://problem/11304699>.
Swift SVN r1570
