c++: Failure to lookup using-decl name [PR98231]

c++: Failure to lookup using-decl name [PR98231]

In r11-4690 we removed the call to finish_nonmember_using_decl in
tsubst_expr/DECL_EXPR in the USING_DECL block.  This was done not
to perform name lookup twice for a non-dependent using-decl, which
sounds sensible.

However, finish_nonmember_using_decl also pushes the decl's bindings
which we still have to do so that we can find the USING_DECL's name
later.  In this case, we've got a USING_DECL N::operator<<  that we are
tsubstituting.  We already looked it up while parsing the template
"foo", and lookup_using_decl stashed the OVERLOAD it found into
USING_DECL_DECLS.  Now we just have to update the IDENTIFIER_BINDING of
the identifier for operator<< with the overload the name is bound to.

I didn't want to export push_local_binding so I've introduced a new
wrapper.

gcc/cp/ChangeLog:

	PR c++/98231
	* name-lookup.c (push_using_decl_bindings): New.
	* name-lookup.h (push_using_decl_bindings): Declare.
	* pt.c (tsubst_expr): Call push_using_decl_bindings.

gcc/testsuite/ChangeLog:

	PR c++/98231
	* g++.dg/lookup/using63.C: New test.