[libc] Migrate to using LIBC_NAMESPACE_DECL for namespace declaration (#98597)

This is a part of #97655.

Revert "[libc] Migrate to using LIBC_NAMESPACE_DECL for namespace declaration" (#98593)

Reverts llvm/llvm-project#98075

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[libc] Migrate to using LIBC_NAMESPACE_DECL for namespace declaration (#98075)

This is a part of #97655.

[libc] Add the implementation of the fdopen function (#94186)

Fixes #93711 .
This patch implements the ``fdopen`` function. Given that ``fdopen``
internally calls ``fcntl``, the implementation of

[libc] Add the implementation of the fdopen function (#94186)

Fixes #93711 .
This patch implements the ``fdopen`` function. Given that ``fdopen``
internally calls ``fcntl``, the implementation of ``fcntl`` has been
moved to the ``__support/OSUtil``, where it serves as an internal public
function.

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