* include/std/variant (__do_visit): Add a template parameter
for enforcing same return types for visit.
(__gen_vtable_impl): Likewise.
(_S_apply_single_alt): Adjust.
(__visit_invoke_impl): New. Handle casting to void.
(__do_visit_invoke): New. Enforces same return types.
(__do_visit_invoke_r): New. Converts return types.
(__visit_invoke): Adjust.
(__gen_vtable):  Add a template parameter for enforcing
same return types for visit.
* testsuite/20_util/variant/visit_r.cc: Add a test for a visitor with
different return types.
* testsuite/20_util/variant/visit_neg.cc: New. Ensures that
visitors with different return types don't accidentally
compile with regular visitation.

From-SVN: r270216