[llvm] [CodeGenPrepare] Prefix option names with cgp- (PR #228838)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Section prefix options are used by Google and Meta, while the misc options, to the best of my knowledge, are niche. 

While we aren't bound by backward compatibility for these internal developer settings, I want to ensure this migration has much less probability to get complaints from Google/Meta.... Section prefix options can be migrated as a follow-up.

https://github.com/llvm/llvm-project/pull/228838