[clang] [C++20][Modules] Restore inliness of constexpr/consteval functions defined in-class (PR #109470)

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https://github.com/cor3ntin commented:

I think this looks reasonable but I'd like @iains or @ChuanqiXu9 to review too.
Can you add a changelog entry?

https://github.com/llvm/llvm-project/pull/109470