Let PassBuilder Expose PassInstrumentationCallbacks

Summary:
This is an effort to allowing external libraries register their own pass instrumentation during their llvmGetPassPluginInfo() calls.

By exposing this through the added getPIC(), now a pass writer can do something like this:

```
extern "C" ::llvm::PassPluginLibraryInfo LLVM_ATTRIBUTE_WEAK
llvmGetPassPluginInfo() {
  return {
    ..,
    [](llvm::PassBuilder &PB) {
      PB.getPIC()->registerAfterPassCallback(move(f));
    }
  };
}
```

Reviewers: chandlerc, philip.pfaffe, fedor.sergeev

Reviewed By: fedor.sergeev

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71086