swift234

Introduce a new TCC_AllowLValue bit to inform 
typeCheckArgumentChildIndependently when we want an lvalue back, 
instead of it always producing an lvalue.  Provide it in a couple of
places, the most interesting of which is when forming a call to an 
operator where an InOutExpr is implicitly provided by the fact that
the operator is an 'assignment' operator (something we don't actually
model right in the language at the moment for unary operators).

Producing this uses the candidate list we get from analyzing the
function, which makes this the first example of using the callee to
provide type information when analyzing an argument subexpression.

All this work for NFC. :-)


Swift SVN r30600