In https://codereview.chromium.org/1757733004/ we had to introduce LLVM_FORCE_LOCAL_BUILD to not download Clang binaries from clang upload bots (as the binaries do not exist at the time when update.py --if-needed called).

Theoretically, clang upload bots could avoid calling gclient sync, and only rely on the primary repository. In this case, LLVM_FORCE_LOCAL_BUILD will not be needed.

Investigate this possibility.