[clang] [CIR] Intitial upstreaming of LibOpt pass (PR #172487)

[Hendrik Hübner via cfe-commits]

HendrikHuebner wrote:

@andykaylor Updated. I also added a flag to enable the lib opt pass in the first place. At some point there should be a discussion of how give users more fine-grained control of which optimizations are done but lets leave that for another PR once we actually have some optimizations.

I'm also getting a CI failure. I don't know how to reproduce it locally, the tests are passing for me. Maybe its related to the CLI flag I added? 

https://github.com/llvm/llvm-project/pull/172487