[llvm] [OptBisect] Add function filtering support to opt-bisect (PR #222123)

[Daniel Donenfeld via llvm-commits]

daniel-donenfeld wrote:

> Could OptBisect.md use a small update for this feature?

The other flags for finer control of opt-bisect are similarly undocumented. Would it make more sense to add documentation to all of them here or in a followup PR? 

https://github.com/llvm/llvm-project/pull/222123