[clang] Reset CodeGenOptions fields for clean module/PCH builds (PR #138256)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

I feel `.clear()` should have a similar semantic with `= {};`. It will be confusing for people to understand this. It will be helpful if you can provide a test to show the difference.

https://github.com/llvm/llvm-project/pull/138256