[clang] [Clang] ASTUnit should use the CompilerInstance's CodeGenOptions (PR #195338)

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Sirraide wrote:

Note: The `CodeGenOptions` stored in a `CompilerInstance` are a `std::shared_ptr`, the ones in the the `ASTUnit` a `std::unique_ptr`, so an alternative here would be to make both a `std::shared_ptr` and then expose the former as a `std::shared_ptr` (we currently only have a getter that returns a `CodeGenOptions&`) and then make both pointers to the same object.

Currently, `CompilerInstance` stores quite a bit of state behind `std::shared_ptr`s, but it doesn’t expose any of them as getters; I’m not sure if there is a reason for that, but I’ve opted for keeping these as two separate objects for now.

https://github.com/llvm/llvm-project/pull/195338