openacc: fix ICE for non-decl expression in non-contiguous array base-pointer

openacc: fix ICE for non-decl expression in non-contiguous array base-pointer

Currently, we do not support cases like struct-members as the base-pointer
for an OpenACC non-contiguous array. Mark such cases as unsupported in the
C/C++ front-ends, instead of ICEing on them.

gcc/c/ChangeLog:

	* c-typeck.c (handle_omp_array_sections_1): Robustify non-contiguous
	array check and reject non-DECL base-pointer cases as unsupported.

gcc/cp/ChangeLog:

	* semantics.c (handle_omp_array_sections_1): Robustify non-contiguous
	array check and reject non-DECL base-pointer cases as unsupported.