[NewPM] Use the default AA pipeline by default

We almost always want to use the default AA pipeline. It's very easy for
users of PassBuilder to forget to customize the AAManager to use the
default AA pipeline (for example, the NewPM C API forgets to do this).

If somebody wants a custom AA pipeline, similar to what is being done
now with the default AA pipeline registration, they can

  FAM.registerPass([&] { return std::move(MyAA); });

before calling

  PB.registerFunctionAnalyses(FAM);

For example, LTOBackend.cpp and NewPMDriver.cpp do this.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D113210