swift234

Assign the result of binary operations to a variable, in preparation for checking its type in my next check-in.  
Ugly (both in layout and functionality) workaround for Gyb rdar://15928178, using operator if/else if/else instead of if/elif statements;
this was fiddly to get working, hence the separate check-in.
Sharpen the expected error string for mixed type operators, and add an expected warning and note for assigning the result of
an assignment, which is righteous since we're not C.
Approved by Dmitri.


Swift SVN r24586