[patch] libstdc++/69293 Use static assertion for uses-allocator construction

[Jonathan Wakely]

The PR is actually due to a defect in the standard, which I reported
today. The reporter said we're missing a check for is_constructible<T,
Args..., inner_allocator_type> that would ensure we go to bullet (9.4)
and make the example in the PR ill-formed. I didn't add that check
because it's redundant, we don't need to check is_constructible we can
just try the construction and if it fails the program is ill-formed
anyway.

However, due to the  defect in the standard the example in the PR
*should* be ill-formed, but isn't.  This patch adds a static assertion
making it ill-formed. It would be ill-formed anyway once the defect is
resolved and we implement the resolution, but with the static
assertion we give a better diagnostic, both for
scoped_allocator_adaptor and for ill-formed uses of allocators with
std::tuple.

Tested powerpc64le-linux, committed to trunk.


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