openmp: Fix C ICE on OpenMP atomics

openmp: Fix C ICE on OpenMP atomics

c_parser_binary_expression was using build2 to create a temporary holder
for binary expression that c_parser_atomic and c_finish_omp_atomic can then
handle.  The latter performs then all the needed checking.

Unfortunately, build2 performs some checking too, e.g. PLUS_EXPR vs.
POINTER_PLUS_EXPR or matching types of the arguments, nothing we can guarantee
at the parsing time.  So we need something like C++ build_min_nt*.  This
patch implements that inline.

2020-11-24  Jakub Jelinek  <jakub@redhat.com>

	PR c/97958
	* c-parser.c (c_parser_binary_expression): For omp atomic binary
	expressions, use make_node instead of build2 to avoid checking build2
	performs.

	* c-c++-common/gomp/pr97958.c: New test.

(cherry picked from commit 2aaf44a90283156ec0e70ad4d9030f3ba5054c6f)