openmp: Fix handling of non-addressable shared scalars in parallel nested inside of target [PR93515]

As the following testcase shows, we need to consider even target to be a construct
that forces not to use copy in/out for shared on parallel inside of the target.
E.g. for parallel nested inside another parallel or host teams, we already avoid
copy in/out and we need to treat target the same.

2020-02-06  Jakub Jelinek  <jakub@redhat.com>

	PR libgomp/93515
	* omp-low.c (use_pointer_for_field): For nested constructs, also
	look for map clauses on target construct.
	(scan_omp_1_stmt) <case GIMPLE_OMP_TARGET>: Bump temporarily
	taskreg_nesting_level.

	* testsuite/libgomp.c-c++-common/pr93515.c: New test.