[mlir-reduce] 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. Although other ops can
now be parsed as top-level, the actual reduction passes are still
restricted to `builtin.module` as it didn't seem straightforward to
update them.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D134242