Since https://github.com/llvm/llvm-project/pull/91150, a proxy header
for the errno macros is available and gets included in `libc_errno.h`
since then.
As `libc_errno.cpp` includes `libc_errno.h`, which already includes the
proxy header `hdr/errno_macros.h`, there's no need to include it in
`libc_errno.cpp` if we are in overlay mode, because the proxy header
takes care to either include our header from libc/include/ (fullbuild)
or the corresponding underlying system header (overlay).
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