As far as I know, there was only ever one version of LLVM at a time in the monorepo. It's possible things changed after I left (either the compiler team or Google).
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.
How does Google deal with other projects' embedded code copies? ie some other project embeds a random old snapshot of LLVM, when you import that project into the monorepo do you end up with two copies of LLVM, or do you strip the old copy and port the codebase to the current LLVM in the monorepo?
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.