Prefer signal from land checks over PR signals (#83715)
# The problem
When a dev forks their branch from a red master build, their branch can fail CI checks for reasons unrelated to their changes, but the same checks would however pass in the land validation commit (which is rebased off of viable/strict)
Today, in the above scenario the `merge -l` command fails because mergebot sees the failing checks in the PR, which is not helpful when that same check passes in land validation.
# The solution
This PR changes the behavior so that:
1. If both the PR and land validation ran a workflow, only look at the results from land validation
2. If only the PR ran a specific workflow (e.g. for CLA Check or a nightly run) then continue to look the result from the PR (which matches existing behavior)
### Bonus fixes
It also includes a few extra BE fixes:
- Replaces the tuple we used to pass workflow check results around with a named tuple so that it's easier to tell what data is being used
- Reduces the number of API calls to github by ~50% during merges. Before, we were pulling results from github every time and then filtering it down to the relevant category of checks (e.g. failed/pending/startup_failed). Now, our filters share the check results
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83715
Approved by: https://github.com/zengk95
