[utils] New script `check_ninja_deps.py`

Summary:
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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83032