Support running with new pass manager

[nikic]

Rustc currently still uses the legacy LLVM pass manager. There's been some discussion about enabling NewPM by default for Clang 10, so we might want to start migrating in that direction as well.

To start with, we'd probably want a debug option -Z llvm-pass-manager=new that is not enabled by default, but can be used to perform initial evaluation.

For reference, this is how newpm use looks like in clang: https://github.com/llvm/llvm-project/blob/90d2be0163e8f84dfc0b73128d873ac8d84c10f8/clang/lib/CodeGen/BackendUtil.cpp#L985

[cuviper]

I'm going to take a shot at this.

[nikic]

Also looking into this now. Seems less hard than I originally expected, though some things can't be supported (e.g. MergeFunctions doesn't seem to support NewPM yet?)

[nikic]

I've created a prototype for newpm at #67954.