[Plugins] Add a slim plugin API to work together with the new PM

Summary:
Add a new plugin API. This closes the gap between pass registration and out-of-tree passes for the new PassManager.

Unlike with the existing API, interaction with a plugin is always
initiated from the tools perspective. I.e., when a plugin is loaded, it
resolves and calls a well-known symbol `llvmGetPassPluginInfo` to obtain
details about the plugin. The fundamental motivation is to get rid of as
many global constructors as possible.  The API exposed by the plugin
info is kept intentionally minimal.

Reviewers: chandlerc

Reviewed By: chandlerc

Subscribers: bollu, grosser, lksbhm, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D35258

llvm-svn: 329273