Other distros like Ubuntu support the installation of multiple versions of
LLVM/Clang side by side.  One of the things Clang is really good at is
support for the most recently approved upcoming features of the C++17
standard.  The best support for testing such features is with the latest
sys-devel/llvm-9999.  However if I want to compile Mesa against a stable
version LLVM/Clang as well, I don't get that option.