This patchset allows us to force depends to use system clang. Previously, #17099 removes our dependency on a specific clang we download from llvm.org, but theuni pointed out that since OSX builds are only ever built with a version of clang that is chosen and "blessed" by Apple, it is more likely that the user will encounter problems if they use their system clang. This patchset forces the user to set FORCE_USE_SYSTEM_CLANG=1 in order to use their system clang (when they know what they're doing)
Backport of core PR17919.