[clang] [C++][Modules] A module directive may only appear as the first preprocessing tokens in a file (PR #144233)

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yronglin wrote:

> Should this commit also mark the papers as implemented on https://clang.llvm.org/cxx_status.html?

This PR is only part of it, so I want to mark it as "implemented" after the paper feature is fully implemented.

https://github.com/llvm/llvm-project/pull/144233