[clang] [NFC] [C++] [Modules] Mark P2788 as implemented and add test (PR #147138)

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yronglin wrote:

Can you add the test from https://github.com/llvm/llvm-project/pull/144214? I think we also need to check that in clang/test/CXX/basic/basic.link/p3.cpp

https://github.com/llvm/llvm-project/pull/147138