[committed] libstdc++: Implement LWG issues 3320, 3369, 3200, 3348, 3237

[Jonathan Wakely]

make polymorphic_allocator throw consistent type (LWG 3237)
Add __cpp_lib_unwrap_ref feature test macro
midpoint should not constrain T is complete (LWG 3200)
span's deduction-guide for built-in arrays doesn't work (LWG 3369)
Remove std::span::cbegin and std::span::cend (LWG 3320)

Tested powerpc64le-linux, committed to master.
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