[mlir] Execute all requested translations in MlirTranslateMain

Currently, MlirTranslateMain only executes one of the requested translations, and does not error if multiple are specified. This commit enables translations to be chained in the specified order.

This makes round-trip tests easier, since existing import/export passes can be reused and no combined round-trip passes have to be registered (example: mlir-translate  -serialize-spirv -deserialize-spirv).

Additionally, by leveraging TranslateRegistration with file-to-file TranslateFunctions, generic pre- and post-processing can be added before/after conversion to/from MLIR.

Reviewed By: lattner, Mogball

Differential Revision: https://reviews.llvm.org/D143719