[clang] [clang][modules] Fix CodeGen options that can affect the AST. (PR #78816)

[Michael Spencer via cfe-commits]

https://github.com/Bigcheese approved this pull request.

LGTM, although are we sure none of the other options should be affecting? I just did a quick search and it seems like this is it.

I don't think it matters here, but the actual option that controls the macro is on `LangOptions` and where it's set has a "FIXME: Eliminate this dependency" where it directly parses the command line. As long as we're not changing the value the `LangOptions` version should always match.

https://github.com/llvm/llvm-project/pull/78816