[clang] [clang][modules] Make -fmodules-decluse work on the public/private pair of modules (PR #192585)

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matts1 wrote:

@Jwata Can you PTAL and review before I send it off to LLVM reviewers.

Do you remember Takuto's github username? Can you also add him here

https://github.com/llvm/llvm-project/pull/192585