c++/modules: Revert "Remove unnecessary lazy_load_pendings"

c++/modules: Revert "Remove unnecessary lazy_load_pendings"

This reverts commit r16-63-g241157eb0858b3.  It turns out that the
'lazy_load_pendings' is necessary if we haven't seen a binding for the
given template name at all in the current TU, as it is also used to find
template instantiations with the given name.

gcc/cp/ChangeLog:

	* name-lookup.cc (lookup_imported_hidden_friend): Add back
	lazy_load_pendings with comment.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/tpl-friend-19_a.C: New test.
	* g++.dg/modules/tpl-friend-19_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>