[clang] [Clang] Add codegen option to add passbuilder callback functions (PR #70171)

[Arthur Eubanks via cfe-commits]

https://github.com/aeubanks commented:

from an internal chat, this is meant to be used by modifying `PassBuilderCallbacks` from a statically linked clang plugin. that makes sense to me, could you update the description with this? as for a test, we can add a test similar to `PluginsOrder`. and then manually verify that statically linking a clang plugin also works for this should be good enough

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