[mlir] make ModuleTranslation mapping fields private

ModuleTranslation contains multiple fields that keep track of the mappings
between various MLIR and LLVM IR components. The original ModuleTranslation
extension model was based on inheritance, with these fields being protected and
thus accessible in the ModuleTranslation and derived classes. The
inheritance-based model doesn't scale to translation of more than one derived
dialect and will be progressively replaced with a more flexible one based on
dialect interfaces and a translation state that is separate from
ModuleTranslation. This change prepares the replacement by making the mappings
private and providing public methods to access them.

Depends On D96436

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D96437