[OpenMP] Permit map with DSA on combined directive

[OpenMP] Permit map with DSA on combined directive

For `map`, the following restriction changed in OpenMP 5.0:

* OpenMP 4.5 [2.15.5.1, Restrictions]: "A list item cannot appear in
  both a map clause and a data-sharing attribute clause on the same
  construct.

* OpenMP 5.0 [2.19.7.1, Restrictions]: "A list item cannot appear in
  both a map clause and a data-sharing attribute clause on the same
  construct unless the construct is a combined construct."

This patch removes this restriction in the case of combined constructs
and OpenMP 5.0, and it updates Sema not to capture a scalar by copy in
the target region when `firstprivate` and `map` appear for that scalar
on a combined target construct.

This patch also adds a fixme to a test that now reveals that a
diagnostic about loop iteration variables is dropped in the case of
OpenMP 5.0.  That bug exists regardless of this patch's changes.

Reviewed By: ABataev, jdoerfert, hfinkel, kkwli0

Differential Revision: https://reviews.llvm.org/D65835

llvm-svn: 369619