Each upstream commit didn't land directly into the monorepo, instead there was a long lived branch, and on the compiler team there was a buildcop rotation responsible for doing an integrate from that branch into //third_party/llvm. This included running the tests (and fixing any problems) for any other software that depends on LLVM as well as building an unstable crosstool and doing some basic smoke tests on that. Taking that crosstool through testing and to stable crosstool was the responsibility of a different buildcop rotation, using a special compiler team tool for testing the testing crosstool nightly, then to release to stable we used the ordinary presubmits, but for all projects at once, making its testing as similar as possible to any normal code change.