Some `LangOptions` duplicate their `CodeGenOptions` counterparts. My
understanding is that this was done solely because some infrastructure
(like preprocessor initialization, serialization, module compatibility
checks, etc.) were only possible/convenient for `LangOptions`. This PR
implements the missing support for `CodeGenOptions`, which makes it
possible to remove some duplicate `LangOptions` fields and simplify the
logic. Motivated by https://github.com/llvm/llvm-project/pull/146342.
This patch tests code in D108976. This split is necessary to avoid temporary regression.
Depends on D108974,
Reviewed By: dexonsmith
Differential Revision: https://reviews.llvm.org/D109158
