[mlir][Pass] Make PassManager default to op-agnostic

Currently `PassManager` defaults to being anchored on `builtin.module`.
Switching the default makes `PassManager` consistent with
`OpPassManager` and avoids the implicit dependency on `builtin.module`.

Specifying the anchor op type isn't strictly necessary when using
explicit nesting (existing pipelines will continue to work), but I've
updated most call sites to specify the anchor since it allows for better
error-checking during pipeline construction.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D137731