[TableGen] DecoderEmitter clean-ups and modernization. (#84832)

The decoder emitter is showing some signs of age. This patch makes a few
kinds of clean-ups:
- Use ranged-for more widely, including using enumerate() for those
loops maintaining a loop index along with the items.
- Reduce the number of arguments to fieldFromInsn (removes an out
reference parameter: CodingStandards). The insn_t argument to insnWithID
can/should probably be removed soon too since modern C++ allows us to
return a local container without a copy.
- Use raw strings for the large emitted code segments. This enhances
both readability and modifiability.