[libc] [llvm] [libc] Move libc_errno.h to libc/src/__support and make LIBC_ERRNO_MODE_SYSTEM to be header-only. (PR #143187)

[via llvm-commits]

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+//===-- Common defines for sharing LLVM libc with LLVM projects -*- C++ -*-===//
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lntue wrote:

Done.

https://github.com/llvm/llvm-project/pull/143187