[libc] [libcxx] [llvm] [libcxx][libc] Hand in Hand PoC with from_chars (PR #91651)

[Mark de Wever via llvm-commits]

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+// -*- C++ -*-
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mordante wrote:

I just did most of these changes, which I did before I saw this comment.

https://github.com/llvm/llvm-project/pull/91651