[clang] [PassBuilder] Add a mechanism for adding passbuilder callbacks for static builds (PR #70171)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I don't think a global variable is the right interface for this; we've been trying to move away from global variables because they cause breakage when LLVM is used in multiple contexts in the same process.

Functions that construct pass pipelines that are usable as libraries can take an array of PassPluginLibraryInfo as an argument.

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