Sometimes patch builds of "debug-compile-coverage" on Darwin fail, with "llvm-cov: not found". Hannes, could you take a look at see if this needs to be a BUILD ticket?:

https://evergreen.mongodb.com/task/mongo_c_driver_darwin_debug_compile_coverage_0f5651ed10b5ad564411a010c8bab06d43ddfd62_17_03_31_15_56_18