The overhead of building clang is way more than building simple LLVM. It does not make sense at all to have clang as part of LLVM. At least there should be an option to tell VCPKG to only build LLVM.

When you have a port named LLVM it should be LLVM, not LLVM/Clang. You can have another package named Clang or you can add config to LLVM package to compile it with Clang.
Whose idea was that? Seriously?