[clang] [Serialization] Downgrade inconsistent flags from erros to warnings (PR #115416)

[Boris Kolpackov via cfe-commits]

boris-kolpackov wrote:

I don't know, I feel like you are just kicking the can down the road: if the user sees a harmless warning continuously, they will suppress this warning, and then won't see the harmful ones in the future. But then again we do this all the time in C++, so I guess it's par for the course.

> after reading your links, I realized you're not against the change but saying build2 is good.

Well, not exactly. We have better chance in `build2` because we control the entire build (unless you use system-installed libraries, but in this case we still build the BMIs). And we thought a bit about build compatibility (in that HOWTO) and what compile options should and should not be specified in library `buildfiles`, which I think will also help with BMI compatibility.

https://github.com/llvm/llvm-project/pull/115416