Make the implicit nesting behavior of the PassManager user-controllable and default...

Make the implicit nesting behavior of the PassManager user-controllable and default to false

This is an error prone behavior, I frequently have ~20 min debugging sessions when I hit
an unexpected implicit nesting. This default makes the C++ API safer for users.

Depends On D90669

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D90671