This is strange, this was’t the case in LLVM 7: How can I setup LLVM 7 as default C / C++ compiler in Bionic - #2 by native-api
And checking http://apt.llvm.org/buster/dists/llvm-toolchain-buster-8/main/binary-amd64/Packages shows that clang-8 does depend on libstdc++-8-dev (but not libc++-8-dev).
Without seeing the build and the codebase, I cannot say more.
Actually, you still can link to the build, but only Travis support staff will be able to open the link.