swift234

Access properties and subscripts in the most efficient
semantically valid way.

Previously, this decision algorithm was repeated in a
bunch of different places, and it was usually expressed
in terms of whether the decl declared any accessor
functions.  There are, however, multiple reasons why a
decl might provide accessor functions that don't require
it to be accessed through them; for example, we
generate trivial accessors for a stored property that
satisfies a protocol requirement, but non-protocol
uses of the property do not need to use them.

As part of this, and in preparation for allowing
get/mutableAddressor combinations, I've gone ahead and
made l-value emission use-sensitive.  This happens to
also optimize loads from observing properties backed
by storage.

rdar://18465527

Swift SVN r22298