swift234

Enhance diagnostic for failed overload resolution with operator calls by
printing the operator, adjusting the diagnostic to point to the operator
and not the second operand, properly highlighting the ranges of the operands,
and printing out the types of the operands when appropriate.

Printing out the types is very useful when trying to understand the mismatch.

Note: I only partially know what I am doing at this stage.  This feels a bit
kludgy, as the "cast<>"s reek of not enough implicit invariants being documented
in the ASTs themselves.

Fixes <rdar://problem/11266762>

Swift SVN r1449