[llvm] [clang] [PassBuilder] Add a mechanism for adding passbuilder callbacks for static builds (PR #70171)

[William Moses via cfe-commits]

wsmoses wrote:

Let me fork off the clang part off into a separate PR from the codegenoption.

For context, I'm in an environment where I can't modify the LLVM build files (but I can import them), but want to produce the modified clang, without maintaining a separate fork just to add the pass.

https://github.com/llvm/llvm-project/pull/70171