[mlir][NFC] Move Translation.h to a Tools/mlir-translate directory

Translation.h is currently awkwardly shoved into the top-level mlir, even though it is
specific to the mlir-translate tool. This commit moves it to a new Tools/mlir-translate
directory, which is intended for libraries used to implement tools. It also splits the
translate registry from the main entry point, to more closely mirror what mlir-opt
does.

Differential Revision: https://reviews.llvm.org/D121026