Fix bug in processing of array dimensions in data clauses.

Fix bug in processing of array dimensions in data clauses.

The g++ front end wraps the array length and low_bound values in
NON_LVALUE_EXPR, causing the subsequent tests for INTEGER_CST to fail.
The test case c-c++-common/goacc/kernels-loop-annotation-1.c was
tickling this bug and giving bogus errors in g++ because it was falling
through to dynamic array code instead of recognizing the constant bounds.

This patch was posted upstream here
https://gcc.gnu.org/pipermail/gcc-patches/2020-March/542694.html
but not yet committed.  It may be that some other fix for this problem
is implemented on mainline instead; check before merging this patch.

2020-03-31  Sandra Loosemore  <sandra@codesourcery.com>

	gcc/cp/
	* semantics.cc (handle_omp_array_sections_1): Call STRIP_NOPS
	on length and low_bound;
	(handle_omp_array_sections): Likewise.