[clang] [clang][Modules] Complete the implementation of P2615: Meaningful exports (PR #194201)

[Victor Chernyakin via cfe-commits]

localspook wrote:

> This will introduce breaking changes. I think it is fine enough to emit warning here.

Do you feel strongly about that? FWIW, GCC and MSVC have both implemented this as an error.

https://github.com/llvm/llvm-project/pull/194201