c++: more checks for exporting names with using-declarations

c++: more checks for exporting names with using-declarations

Currently only functions are directly checked for validity when
exporting via a using-declaration.  This patch also checks exporting
non-external names of variables, types, and enumerators.  This also
prevents ICEs with `export using enum` for internal-linkage enums.

While we're at it this patch also improves the error messages for these
cases to provide more context about what went wrong.

gcc/cp/ChangeLog:

	* name-lookup.cc (check_can_export_using_decl): New.
	(do_nonmember_using_decl): Use above to check if names can be
	exported.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/using-10.C: New test.
	* g++.dg/modules/using-enum-2.C: New test.

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