Rethink lookup of `clang-$VERSION` in CMake

Currently, our chained lookup for LLVM tools looks something like this:

find_program(CMAKE_CXX_COMPILER clang++-16)
find_program(CMAKE_CXX_COMPILER clang++-15)
find_program(CMAKE_CXX_COMPILER clang++-14)
find_program(CMAKE_CXX_COMPILER clang++-13)
find_program(CMAKE_CXX_COMPILER clang++-12)
find_program(CMAKE_CXX_COMPILER clang++-11)
find_program(CMAKE_CXX_COMPILER clang++)

This usually works fine for globally-installed versions of LLVM, but breaks down if I want to point the build system at a local build[^1]. We should more consistently set the desired LLVM version, or use some of the options to find_program if possible.

Not a huge priority, as for this use-case I can just fiddle with the code, but it would be good to fix.

[^1]: For context, this is related to #730 - the changes to pointer semantics are requiring more of a debugging process than we've needed for other version changes.

Start by searching the CMake configuration for the chained find_program calls that select clang++-16 through clang++. Read the relevant CMake find_program options and reproduce the local LLVM build scenario described in the issue; done means the desired LLVM version can be selected consistently without manually editing the lookup chain.