[llvm-branch-commits] [LLVM][NFC] Move PassPlugin from Passes to Extensions lib (PR #172478)

[Alexis Engelke via llvm-branch-commits]

https://github.com/aengelke created https://github.com/llvm/llvm-project/pull/172478

This avoid pulling in the entire Passes library with all passes as
dependencies when just referring to PassPlugin, which is in fact
independent of the Passes themselves.

Reviwwers: nikic, vgvassilev, serge-sans-paille, efriedma-quic