[clang] [clang-tools-extra] [clang][modules] Serialize `CodeGenOptions` (PR #146422)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

> My concern is, there are many other CodeGenOpts. And if we model them as incompatible default, it will be a breaking change for existing users. We don't like breaking change. How do you feel about to emit a warning so that the end users can have a chance to escape?

I'm not suggesting to treat `CodeGenOptions` as incompatible by default. For now I'm just trying to remove the duplication and improve the infrastructure.`CodeGenOptions` are still benign by default, and can only be marked as compatible, which doesn't have any impact on explicitly-built modules. This PR is intended to be an NFC.

I'm open to having a diagnostic that warns about mismatch in compatible `CodeGenOptions` for explicitly-built modules, but I don't think we should block this PR on that.

https://github.com/llvm/llvm-project/pull/146422