[llvm-branch-commits] [Clang] Load pass plugins before parsing LLVM options (PR #171868)

[Alexis Engelke via llvm-branch-commits]

https://github.com/aengelke created https://github.com/llvm/llvm-project/pull/171868

This permits pass plugins to use llvm::cl::opt. Additionally, add a test
of -fpass-plugin, this was previously not tested at all.

I'm not sure whether using the LLVM Bye.so in the tests is possible this
way (e.g., if Clang is built standalone).