[mlir] Simplify ModuleTranslation for LLVM IR

A series of preceding patches changed the mechanism for translating MLIR to
LLVM IR to use dialect interface with delayed registration. It is no longer
necessary for specific dialects to derive from ModuleTranslation. Remove all
virtual methods from ModuleTranslation and factor out the entry point to be a
free function.

Also perform some cleanups in ModuleTranslation internals.

Depends On D96774

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D96775