[libc++] Enable missing allocator constructors in C++11 (#204845)
LWG2210 was a patch to N2554 (C++11) which added missing
allocator-extended constructors to various containers. When LWG2210 was
implemented, the constructors were gated with C++14. However, we
normally apply LWG issue resolutions as DRs.
This patch enables these constructors in C++11 mode too.
Fixes #204843
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Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
Co-authored-by: A. Jiang <de34@live.cn>