[MLIR] Add LLVMConversionTarget as a customization point. NFC.

This is in preparation for the next patch D75141. The purpose is to
provide a single place where LLVM dialect registers its ops as
legal/illegal.

Reviewers: ftynse, mravishankar, herhut

Subscribers: jholewinski, bixia, sanjoy.google, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, csigg, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, Joonsoo, llvm-commits

Differential Revision: https://reviews.llvm.org/D75140