[NFC][regalloc] Introduce RegAllocGreedy.h

This was suggested in D114831. It should simplify the relation between
eviction advisor and the allocator, and simplify ingesting more features
tied to the internals of the allocator, in the future.

This change simply pulls out RAGreedy, places it in the llvm namespace,
and cleans up a bit the includes in the new header file.

Differential Revision: https://reviews.llvm.org/D116114