openmp: Change the way of building of reduction array type

The PR98597 patch regresses on _Atomic-3.c, as in the C FE building an
array type with qualified elements results in a type incompatible with
when an array type with unqualified elements is qualified afterwards.
This patch adds a workaround for that.

2021-01-15  Jakub Jelinek  <jakub@redhat.com>

	* c-typeck.c (c_finish_omp_clauses): For reduction build array with
	unqualified element type and then call c_build_qualified_type on the
	ARRAY_TYPE.