[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 support for `powi` as an LLVM libc extension on the GPU (#98236)

Summary:
This function is used by the CUDA / HIP / OpenMP headers and exists as
an NVIDIA extension basically. This functi

[libc] Add support for `powi` as an LLVM libc extension on the GPU (#98236)

Summary:
This function is used by the CUDA / HIP / OpenMP headers and exists as
an NVIDIA extension basically. This function is implemented in the C23
standard as `pown`, but for now we need to provide `powi` for backwards
compatibility. In the future this entrypoint will just be a redirect to
`pown` once that is implemented.

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