update pipeline generation script

- Add control over "modified files" set and agent targeting rules. Mostly for testing purposes.

- Moved sections that outputs "steps" after functions

- Added step that report back to Phabricator

That will allow to replace step generation from invocation of script https://github.com/google/llvm-premerge-checks/blob/main/scripts/pipeline_premerge.py to plain call to ".ci/generate-buildkite-pipeline-premerge".

Differential Revision: https://reviews.llvm.org/D159280