openmp: Fix ICE in omp_discover_declare_target_tgt_fn_r

openmp: Fix ICE in omp_discover_declare_target_tgt_fn_r

This ICEs because node->alias_target is (not yet) a FUNCTION_DECL, but
IDENTIFIER_NODE.

I guess we should retry the discovery before LTO streaming out, the reason
to do it this early is that it can affect the gimplification and omp lowering.

2020-10-06  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/97289
	* omp-offload.c (omp_discover_declare_target_tgt_fn_r): Only follow
	node->alias_target if it is a FUNCTION_DECL.

	* c-c++-common/gomp/pr97289.c: New test.