<rdar://problem/13584715> Converted constant expressions are expected to have integral values.

We were assuming that any expression used as a converted constant
expression would either not have a folded constant value or would be
an integer, which is not the case for some ill-formed constant
expressions. Because converted constant expressions are only used
where integral values are expected, we can simply treat this as an
error path. If that ever changes, we'll need to widen the interface of
Sema::CheckConvertedConstantExpression() anyway.