[openacc] Add dialect definition for acc declare

A declare directive is used to specify the creation of a visible device
copy of a variable for the duration of the implicit data region as it
relates to the scope in which the variable is declared.

In order to support this, the following new operations were added:
1) `acc.global_ctor` and `acc.global_dtor`. These are used whenever the
declare directive applies to a global.
2) `acc.declare_enter` and `acc.declare_exit`. These operations are
modeled similarly to `acc.enter_data` and `acc.exit_data`. The reason
they are not modeled like `acc.data` is so that these operations can be
used both for globals and regions like functions.
3) `acc.declare_device_resident` and `acc.declare_link`. These
operations are modeled in a manner consistent with previously defined
data entry operation model.

The `acc.getdeviceptr` was generalized so that it can be used with
acc.declare_exit.

Reviewed By: clementval, vzakhari

Differential Revision: https://reviews.llvm.org/D155322