[v3 PATCH] Don't revisit a variant we are already visiting.

[Ville Voutilainen]

This shaves off some unnecessary codegen. In the assignment
operators and swap, we are already visiting the rhs, so we shouldn't
visit it again to get to its guts, we already have them in-hand.

2019-03-28  Ville Voutilainen  <ville.voutilainen@gmail.com>

    Don't revisit a variant we are already visiting.
    * include/std/variant (__variant_construct_single): New.
    (__variant_construct): Use it.
    (_M_destructive_move): Likewise, turn into a template.
    (_M_destructive_copy): Likewise.
    (_Copy_assign_base::operator=): Adjust.
    (_Move_assign_base::operator=): Likewise.
    (swap): Likewise.
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