[mlir-translate] 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.

The translation APIs are also updated to take/return 'Operation*'
instead of 'ModuleOp', to allow other operation types to be used. To
simplify translations which are restricted to specific operation types,
'TranslateFromMLIRRegistration' has an overload which performs the
necessary cast and error checking.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D134237