swift234

[CodeCompletion] Add initial support for completing infix operators

This allows us to start code-completing infix operators in postfix
expressions.  As a first step, this patch only handles completing
against the immediate LHS (so for example 1 == 1 <here> doesn't suggest
boolean operators yet).

The next step is to feed the leading sequence expression from the parser
in so we can consider how the operator being completed fits into the
whole sequence expression.

For rdar://problem/22460167

Swift SVN r32465