[MLIR][NFC] Organize the ordering in LLVM.h

Summary:
Order classes by purpose and alphabetically to make it slightly easier
to read through the file.

Reviewers: ftynse!

Subscribers: mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, grosul1, frgossen, Kayjukh, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78914