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

[Junji Watanabe via cfe-commits]

Jwata wrote:

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

LGTM. Let's get feedback from the code owners.

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

@atetubou PTAL

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