[TableGen][CodeEmitter] Introducing the VarLenCodeEmitterGen infrastructure

Full write up:
https://gist.github.com/mshockwave/66e98d099256deefc062633909bb7b5b

The existing CodeEmitterGen infrastructure is unable to generate encoder
function for ISAs with variable-length instructions. This patch
introduces a new infrastructure to support variable-length instruction
encoding, including a new TableGen syntax for writing instruction
encoding directives and a new TableGen backend component,
VarLenCodeEmitterGen, built on top of CodeEmitterGen.

Differential Revision: https://reviews.llvm.org/D115128