[mlir-opt] Support parsing operations other than 'builtin.module' as top-level

This adds a `--no-implicit-module` option, which disables the insertion
of a top-level `builtin.module` during parsing. In this mode any op may
be top-level, however it's required that there be exactly one top-level
op in the source.

`parseSource{File,String}` now support `Operation *` as the container op
type, which disables the top-level-op-insertion behaviour.

Following patches will add the same option to the other tools as well.

Depends on D133644

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D133645