[utils] New script check_ninja_deps.py
This can be run after a ninja-based build, and analyzes the ninja
build files and dependency database to spot any missing dependencies
in the build scripts.
I wrote it in the course of investigating D82659, and it seems likely
to be useful again.
Reviewers: thakis, chandlerc, theraven
Reviewed By: thakis
Subscribers: riccibruno, llvm-commits
Differential Revision: https://reviews.llvm.org/D83032