[clang] Downgrade error to warning (PR #117840)

[via cfe-commits]

https://github.com/cor3ntin requested changes to this pull request.

I don't think the discussion here has run its course
https://github.com/llvm/llvm-project/pull/115416

In particular, I agree with @boris-kolpackov that this is likely to lead to hard-to understand ODR violations issues, which is the exact things modules were meant to solve.

_If_ there was evidence of deployment challenges, we could consider a warning-defaulting-to-an-error, but no less, imo.

(But then again, deployment issues in a projects that mixes compilation flags is expected and it's very unclear to be there is  value in deploying modules without first ensuring a consistent build environment)

@Bigcheese @AaronBallman  


https://github.com/llvm/llvm-project/pull/117840