[OpenMP] Implement TR8 `present` map type modifier in Clang (1/2)

This patch implements Clang front end support for the OpenMP TR8
`present` map type modifier.  The next patch in this series implements
OpenMP runtime support.

This patch does not attempt to implement TR8 sec. 2.22.7.1 "map
Clause", p. 319, L14-16:

> If a map clause with a present map-type-modifier is present in a map
> clause, then the effect of the clause is ordered before all other
> map clauses that do not have the present modifier.

Compare to L10-11, which Clang does not appear to implement yet:

> For a given construct, the effect of a map clause with the to, from,
> or tofrom map-type is ordered before the effect of a map clause with
> the alloc, release, or delete map-type.

This patch also does not implement the `present` implicit-behavior for
`defaultmap` or the `present` motion-modifier for `target update`.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D83061