openmp: Fix ICE on #pragma omp parallel master in template [PR94477]

openmp: Fix ICE on #pragma omp parallel master in template [PR94477]

The following testcase ICEs, because for parallel combined with some
other construct we initialize the omp_parallel_combined_clauses pointer
and expect the construct combined with it to clear it after it no longer
needs it, but OMP_MASTER didn't do that.

2020-04-04  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94477
	* pt.c (tsubst_expr) <case OMP_MASTER>: Clear
	omp_parallel_combined_clauses.

	* g++.dg/gomp/pr94477.C: New test.