In Swift 5 mode, CSGen generates a conversion constraint from
the type of OptionalTryExpr's subexpression, call it T, and
Optional<$tv> for a new type variable $tv.
When applying the solution, we would coerce the sub-expression
to T? if T was not optional, or T otherwise. This was wrong
because there's no reason that $tv is actually equal to T.
Instead, we must coerce the sub-expression to Optional<$tv>,
which is the type of the overall OptionalTryExpr.
Fixes <rdar://problem/46742002>.
