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

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tomasz-kaminski-sonarsource wrote:

I tested this on 19.0.1 and missed the fact that it was fixed on main by https://github.com/llvm/llvm-project/commit/74ac96ae1a81c7ecc0e27ff6f45309cff1f2df97.
I will rework this in NFC comment, that would add tests.

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