c++: ICE with USING_DECL redeclaration [PR98687]

c++: ICE with USING_DECL redeclaration [PR98687]

My recent patch that introduced push_using_decl_bindings didn't
handle USING_DECL redeclaration, therefore things broke.  This patch
amends that by breaking out a part of finish_nonmember_using_decl
out to a separate function, push_using_decl_bindings, and calling it.
It needs an overload, because name_lookup is only available inside
of name-lookup.c.

gcc/cp/ChangeLog:

	PR c++/98687
	* name-lookup.c (push_using_decl_bindings): New, broken out of...
	(finish_nonmember_using_decl): ...here.
	* name-lookup.h (push_using_decl_bindings): Update declaration.
	* pt.c (tsubst_expr): Update the call to push_using_decl_bindings.

gcc/testsuite/ChangeLog:

	PR c++/98687
	* g++.dg/lookup/using64.C: New test.
	* g++.dg/lookup/using65.C: New test.