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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

It should be possible to statically link a pass plugin without modifying any LLVM/clang build files.  polly uses this infrastructure.  (polly is actually referenced explicitly in a couple places, but only to pull it into the build in the first place; you can do something equivalent with LLVM_EXTERNAL_PROJECTS.)

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