[NFC][CodeGen] Add comments for SDNode debug ID

Normally, we place fields serving for debug purpose declarations
under `#if LLVM_ENABLE_ABI_BREAKING_CHECKS`. For `SDNode::PersistentId` and
`SelectionDAG::NextPersistentId`, we do not want to do so because it adds
unneeded complexity without noticeable benefits (see discussion with @thakis
in D120714). This patch adds comments describing why we don't place those
fields under `#if` not to confuse anyone more.

Differential Revision: https://reviews.llvm.org/D123238