Fix omp-low ICE for indirect references based off component access [PR103642]

This issue was triggered after the patch extending syntax for component access
in map clauses in commit 0ab29cf0.

In gimplify_scan_omp_clauses, the case for handling indirect accesses (which
creates firstprivate ptr and zero-length array section map for such decls) was
erroneously went into for non-pointer cases (here being the base struct decl),
so added the
appropriate checks there.

Added new testcase is a compile only test for the ICE. The original omptests
t-partial-struct test actually should not execute correctly, because for
map(t.s->a[:N]), map(t.s[:1]) is not implicitly mapped, thus the entire
offloaded access does not work as is (fixing that omptests test is out of
scope here).

2022-01-27  Chung-Lin Tang  <cltang@codesourcery.com>

	PR middle-end/103642

gcc/ChangeLog:

	* gimplify.cc (gimplify_scan_omp_clauses): Do not do indir_p handling
	for non-pointer or non-reference-to-pointer cases.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/pr103642.c: New test.