swift234

SILGen: Fix materializeForSet for observing accessors

The following patch introduced a regression where materializeForSet
did not invoke didSet/willSet for stored properties with observers:

<https://github.com/apple/swift/commit/249242b08df1c9adc75ea5571ee1be7593945ed4>

The old SILGen materializeForSet was only used for witness thunks,
where it used Ordinary access semantics for the storage.

When I switched over to using the new materializeForSet implementation
for static thunks too, I added some logic to use DirectToStorage access
semantics where appropriate, however this was wrong for observable
properties, which need DirectToAccessor semantics.

Fixes <rdar://problem/24315392>.