swift234

Teach SILGen to take advantage of materializeForSet
accessors on non-dynamic storage.

This allows us to take advantage of dynamic knowledge
that a property is implemented as stored in order to
gain direct access to it instead of copying into a
temporary.  When a class or protocol-member property
is expensive to copy, and particularly when it's of
copy-on-write type, such direct accesses can massively
improve the performance of a program.

We are currently only able to apply this when the
property is implemented purely as stored.  A willSet
observer inherently blocks the optimization, but we
should be able to make it work even for properties
with didSet observers.  This could be done by returning
an arbitrary callback from materializeForSet instead
of returning a flag indicating whether to call the
setter.

rdar://17416120

Swift SVN r22122