When =sys-devel/clang-4.0.0-r2 is installed, all binaries are in directory /usr/lib64/llvm/4/bin but none of them are in /usr/bin.
This is by design. Otherwise you couldn't have multiple slots installed alongside. The slotted directories are added to PATH. You may need to source /etc/profile after installing to get PATHs updated.
Oh yeah, sorry you are right!