Previously, #151360 implemented 'private' clause lowering, but didn't
properly initialize the variables. This patch adds that behavior to make
sure we correctly get the constructor or other init called.
The private clause is the first with 'recipes', so a lot of
infrastructure is included here, including some MLIR dialect changes
that allow simple adding of a privatization. We'll likely get similar
for firstprivate and reduction.
Also, we have quite a bit of infrastructure in clause lowering to make
sure we have most cases we could think of covered.
At the moment, ONLY private is implemented, so all it requires is an
'init' segment (that doesn't call any copy operations), and potentially
a 'destroy' segment. However, actually calling 'init' functions on each
of the elements in them are not properly implemented, and will be in a
followup patch.
This patch implements all of that, and adds tests in a way that will be
useful for firstprivate as well.
