[flang][openacc] Update the global ctor for descriptor

The global ctor for acc declare when the variable is a descriptor
is treated differently. The descriptor is implicity copied in.
An additional registering function will be generated to deal with
the data pointer when the data is actually allocated. This will come in
a follow up patch.

The descriptor is not a user visible detail but an implementation detail.
The intent for declare is that the lifetime is implicitly managed - and the
data must be on device. Since descriptor holds pointer to the data,
it makes sense to also make this available on device at same time.
Copyin is used because it contains relevant details about the data such
as bounds.

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D157338