Auto merge of rust-lang#15727 - DaniPopes:lintgen-unstable-book, r=HKalbasi

feat: generate descriptors for all unstable features

Most unstable features don't have their own chapter in the unstable book, so a rustc helper tool (`src/tools/unstable-book-gen`) generates shims to fill the gaps.
Run this tool to generate the full unstable-book source before parsing it.

Author: bors