c++: Ensure OpenMP reduction with reference type references complete type [PR101516]

c++: Ensure OpenMP reduction with reference type references complete type [PR101516]

The following testcase ICEs because we haven't verified if reduction decl
has reference type that TREE_TYPE of the reference is a complete type,
require_complete_type on the decl doesn't ensure that.

2021-07-21  Jakub Jelinek  <jakub@redhat.com>

	PR c++/101516
	* semantics.c (finish_omp_reduction_clause): Also call
	complete_type_or_else and return true if it fails.

	* g++.dg/gomp/pr101516.C: New test.

(cherry picked from commit aea199f96cf116ba4c81426207acde371556610c)