[patch] libstdc++/69222 Prevent recursive instantiation in std::function

[Jonathan Wakely]

This fixes PR 69222 and PR 69005 for gcc-5-branch, by ensuring we
don't try to determine the result of invoking the function(Functor)
constructor argument when the type is incomplete (because that might
require instantiating the constructor again, which recurses).

Jason fixed 69005 on trunk by making the front end skip that
constructor when performing overload resolution for copy construction,
because it cannot be instantiated to make a copy, but there is still a
problem on the branch, so I'm fixing it in the library. I'm making the
same change on trunk, because it's an improvement anyway.

Tested x86_64-linux, committed to trunk and gcc-5-branch.

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