[libcxx] [lld] [flang] [llvm] [clang] [libc++][streams] P1759R6: Native handles and file streams (PR #76632)

[Mark de Wever via llvm-commits]

https://github.com/mordante requested changes to this pull request.

I did a full review. There are a number of comments, once these are addressed the patch is ready.

https://github.com/llvm/llvm-project/pull/76632