[PM] Enable registration of out-of-tree passes with PassBuilder

[PM] Enable registration of out-of-tree passes with PassBuilder

Summary:
This patch adds a callback registration API to the PassBuilder,
enabling registering out-of-tree passes with it.

Through the Callback API, callers may register callbacks with the
various stages at which passes are added into pass managers, including
parsing of a pass pipeline as well as at extension points within the
default -O pipelines.

Registering utilities like `require<>` and `invalidate<>` needs to be
handled manually by the caller, but a helper is provided.

Additionally, adding passes at pipeline extension points is exposed
through the opt tool. This patch adds a `-passes-ep-X` commandline
option for every extension point X, which opt parses into pipelines
inserted into that extension point.

Reviewers: chandlerc

Reviewed By: chandlerc

Subscribers: lksbhm, grosser, davide, mehdi_amini, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D33464

llvm-svn: 307532