[mlir] Refactor and cleanup the translation facilities.

Summary:
This revision performs several cleanups on the translation infra:
* Removes the TranslateCLParser library and consolidates into Translation
  - This was a weird library that existed in Support, and didn't really justify being a standalone library.
* Cleans up the internal registration and consolidates all of the translation functions within one registry.

Differential Revision: https://reviews.llvm.org/D77514