[clang] [clang][CodeGen] Omit pre-opt link when post-opt is link requested (PR #85672)

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b-sumner wrote:

> > I'm thinking of an option that developers can use. If -link-builtin-bitcodes-postopt, becomes the default, how can developers disable it?
> 
> Presumably you'd add `--no-link-builtin-bitcodes-postopt` and then use `Args.hasFlag(PosFlag, NegFlag, Default)`.

I assume I'd need an -Xclang or -Wl, in front?  And it would need some test coverage.

https://github.com/llvm/llvm-project/pull/85672