I found this "hack" in debian/ubuntu llvm-toolchain source package:

override_dh_strip:
: # running out of diskspace on the buildds
find $(TARGET_BUILD) -name '*.o' -o -name '*.a' | xargs -r rm -f

Apparently, the build bot runs out of space while building, so their solution was to delete the object files (since it isn't needed any more at the time of stripping the binaries).