Add a build-cleaner tool.

[hzeller]

This runs the depend-on-what-you-use command of bant https://github.com/hzeller/bant#dwyu--depend-on-what-you-use

It should be possible to make new cc_libraries(), add a bunch of srcs=[]i and ihdrs=[]i to it, and have bant figure out all the dependencies needed, emitting an edit script to make it happen.

[gemini-code-assist]

Code Review

This pull request introduces a new bash script etc/run-build-cleaner.sh to run the bant build cleaner for optimizing dependencies in cc_libraries. Feedback on the script includes exporting the BAZEL variable so child processes can inherit it, quoting dirname "$0" to handle paths with spaces, and double-quoting the positional parameters "$@" to prevent word splitting and globbing issues.

[hzeller]

To use without annoyance

• Explicitly mark dependencies as keep for bracket-headers used. OpenSTA#365 needs to be submodule updated, so that bant doesn't suggest changes in src/sta/
• Since the project is not fully clean yet (cyclic dependencies), just running all suggested edits will result in bazel complaining about these. This requires first some more progress in bazel: Fix dependencies to not need to disable layering_check #10478 and bazel: Multiple targets using the same source or exporting the same header. #10409
  (but most edits can probably already applied
• To understand what is happending for certain targets, just calling it with verbosity-level 3 (-vvv) will show what is going on under the hood: matching which headers it finds to which libraries, e.g.

etc/run-build-cleaner.sh //src/cgt -vvv

[maliberty]

@dsengupta0628 should update STA later this week so let's hold this PR for that.