[CI] Enable Build Failure Reporting
This patch finishes up the plumbing so that generate_test_report will dump build
failures into the Github checks summary.
Reviewers: Keenuts, lnihlen, dschuff, gburgessiv, cmtice, DavidSpickett
Reviewed By: cmtice, DavidSpickett
Pull Request: https://github.com/llvm/llvm-project/pull/152622
