openmp: Fix up omp_declare_variant{s,_alt} htab handling

openmp: Fix up omp_declare_variant{s,_alt} htab handling

This patch fixes a GC ICE.  During debugging, I've found that during
gimplification we can actually call omp_resolve_declare_variant multiple
times and it would create a new magic declare_variant_alt FUNCTION_DECL
each time, which is undesirable, once we have such a decl, we should just
use that.  The other problem is that there was no cgraph node removal hook.
As the omp_declare_variants htab is used just early during gimplification,
we can just clear the whole htab, rather than trying to lookup and remove
a particular entry.  The other hash table is used later as well and that
one uses just DECL_UID as hash, so in that case the patch removes the elt.

2020-05-27  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/95315
	* omp-general.c (omp_declare_variant_remove_hook): New function.
	(omp_resolve_declare_variant): Always return base if it is already
	declare_variant_alt magic decl itself.  Register
	omp_declare_variant_remove_hook as cgraph node removal hook.

	* gcc.dg/gomp/pr95315.c: New test.