swift234

More groundwork for further improvements to box-to-stack promotion.

Drill down into partial_apply to examine how the container pointer is
used within the partial_apply. If the uses are not unexpected, and do
not allow the container pointer to escape the partial_apply, then we'll
check how the partial_apply is used when passed into an apply by
drilling down one level into that apply (but no further). If the
partial_apply itself cannot escape the current function or any funtion
it is passed to, then we should be able to clone the partial_apply body
and rewrite it to remove the box container pointer (coming in a future
commit).

This is all effectively disabled now by passing false to the call to
canValueEscape in findUnexpectedBoxUse which disables drilling down into
the apply.

Swift SVN r15591