c++ modules: ICE with templated friend and std namespace [PR100134]

The function depset::hash::add_binding_entity has an assert verifying
that if a namespace contains an exported entity, then the namespace must
have been opened in the module purview:

  if (data->hash->add_namespace_entities (decl, data->partitions))
    {
      /* It contains an exported thing, so it is exported.  */
      gcc_checking_assert (DECL_MODULE_PURVIEW_P (decl));
      DECL_MODULE_EXPORT_P (decl) = true;
    }

We're tripping over this assert in the below testcase because by
instantiating and exporting std::A<int>, we in turn define and export the
hidden friend std::f(A<int>) without ever having opened the enclosing
namespace std within the module purview, and thus DECL_MODULE_PURVIEW_P
for std is false.

It's important that the enclosing namespace is std here: if we use
a different namespace then the ICE disappears.  This probably has
something to do with us predefining std via push_namespace from
cxx_init_decl_processing (which makes it look like we've opened it
within the TU), whereas with another namespace we would instead lazily
create its NAMESPACE_DECL from add_imported_namespace.

Since templated friend functions are special in that they give us a way
to introduce a namespace-scope function without having to explicitly
open the namespace, this patch proposes to fix this ICE by propagating
DECL_MODULE_PURVIEW_P from the introduced function to the enclosing
namespace during tsubst_friend_function.

	PR c++/100134

gcc/cp/ChangeLog:

	* pt.cc (tsubst_friend_function): Propagate DECL_MODULE_PURVIEW_P
	from the introduced namespace-scope function to the namespace.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/tpl-friend-8_a.H: New test.
	* g++.dg/modules/tpl-friend-8_b.C: New test.