[clang][dataflow] Add bugprone-dataflow-dead-code check

[Discookie]

Finds instances of always-true and always-false conditions in branch statements, a straightforward use case for the dataflow framework.

This check is a work in progress - the core of the check is simple, but there are too many false positives currently for the check to be usable.
More heuristics are needed to reduce the count of false positives.

Currently only assert-like statements are filtered out. More information on false-positive types are in the docs here:
https://github.com/Discookie/llvm-project/blob/dataflow-deadcode/clang-tools-extra/docs/clang-tidy/checks/bugprone/dataflow-dead-code.rst

Results on some open-source projects

Framework improvements needed for the check to be fully functional:

• A branch transfer function in the Diagnoser entity. The workaround for this missing function is wonky and causes many false positives.
• Some kind of support for C++ classes. I can technically work this around inside my check, but first-class support for it would be nice.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.