openmp: Map holds clause to IFN_ASSUME for C/C++

openmp: Map holds clause to IFN_ASSUME for C/C++

Now that [[assume (cond)]] support is in, this simple patch makes
  #pragma omp assume holds(cond)
use it.

2022-10-06  Jakub Jelinek  <jakub@redhat.com>

	* c-parser.cc (c_parser_omp_assumption_clauses): Emit IFN_ASSUME
	call for holds clause on assume construct.

	* parser.cc (cp_parser_omp_assumption_clauses): Emit IFN_ASSUME
	call for holds clause on assume construct.

	* c-c++-common/gomp/assume-4.c: New test.