c++: repeated export using

c++: repeated export using

A sample implementation of module std was breaking because the exports
included 'using std::operator&' twice.  Since Nathaniel's r15-964 for
PR114867, the first using added an extra instance of each function that was
revealed/exported by that using, resulting in duplicates for
lookup_maybe_add to dedup.  But if the duplicate is the first thing in the
list, lookup_add doesn't make an OVERLOAD, so trying to set OVL_USING_P
crashes.  Fixed by using ovl_make in the case where we want to set the flag.

gcc/cp/ChangeLog:

	* tree.cc (lookup_maybe_add): Use ovl_make when setting OVL_USING_P.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/using-21_a.C: New test.