swift234

Teach CSDiag how to recursively walk into an arbitrary expression, to produce 
its diagnostics in post-order.  Notably, this picks up support for if-expr, which
gives up much better diagnostics in ternary operators.  For example, rdar://17224804
used to produce:

error: could not find an overload for '<=' that accepts the supplied arguments
var monthString = (monthNumber <= 9) ? ("0" + monthNumber) : String(monthNumber)
                  ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

now we produce:

 error: binary operator '+' cannot be applied to operands of type 'String' and 'Int'
var monthString = (monthNumber <= 9) ? ("0" + monthNumber) : String(monthNumber)
                                        ~~~ ^ ~~~~~~~~~~~
 note: overloads for '+' exist with these partially matching parameter lists: (Int, Int), (String, String), (UnsafeMutablePointer<Memory>, Int), (UnsafePointer<Memory>, Int)
var monthString = (monthNumber <= 9) ? ("0" + monthNumber) : String(monthNumber)
                                        ^

which is the correct diagnostic.  While I'm at it, improve the location info for this
binary operator diagnostic to point to the right spot (the operator) and highlight
the LHS/RHS of the operator.



Swift SVN r29774