This patch adds the necessary plumbing to cmake to build the sources related to GlobalISel.
To be able to test this plumbing, this patch also adds an empty implementation of the IRTranslator.
Do not waste time on this empty implementation, the API will likely evolve as the prototype goes.
After this patch lands, it will be possible to build sources related to GlobalISel by adding -DBUILD_GLOBAL_ISEL=ON. By default, this is OFF, thus GlobalISel sources will not impact people that do not explicitly opt-in.
OK to commit?