[clang] [Serialization] Downgrade error to warning for inconsistent language flags (PR #117840)

[Ben Boeckel via cfe-commits]

mathstuf wrote:

> Really we would have to remove the preprocessor from C++ to be able to get away from this.

:clap: Maybe we can get a magic `-fno-preprocess` flag to say "I use no preprocessor shenanigans" and make always-compatible BMIs? Maybe then we could adapt the Fortran/ASM convention where an uppercase extension means "I need preprocessing" and lowercase doesn't. (tongue lightly in cheek…)

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