[openacc] fix an ICE with acc declared VLAs

[Cesar Philippidis]

There's a bug with ACC DECLARE clauses involving VLA variables that
causes lower_omp_target to thow an ICE on VLA decls. The problem
occurred because those clauses are never gimplified. This patch resolves
that ICE by teaching gimplify_oacc_declare how to gimplify them.

It turns out that gimplify_adjust_omp_clauses removes the
gimplify_omp_ctx created by gimplify_scan_omp_clauses. To workaround
that problem, I just taught gimplify_oacc_declare to ensure that each
gimplified decl has an "oacc declare target" attribute. This seems like
a reasonable solution because acc declare is an executable directive,
and hence really doesn't have a normal scope like ACC DATA. The declare
code does install destructors to unmap those clauses (except for those
vars which were ACC DECLARE CREATE'd -- but that's a problem for patch),
but that doesn't depend on the gimplify omp ctx. The only other major
thing that had to be updated was oacc_default_clause, because
omp_notice_variable no longer has a gimple omp ctx to inspect for
existing variables. But that's where the "oacc declare target" attribute
comes into play.

Is this patch OK for trunk?

Cesar
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