revert `$(llvm-config --bindir)/clang++` to get the cpp compiler

A few reports of either people not having this clang++ or having it but
it not finding system headers. On top of that, the reason we added this
in the first place was a bug that surfaced on clang-18 which we've since
fixed.

Order will now be clang++ from path, then the llvm bindir clang++, then an
error.

This can all still be overwritten with `CXX=blah make` like before.