devel/edi: add an option to build the port against libclang

This enables code autocompletion and inline error reporting.
Allow to override LLVM_DEFAULT: contemporary LLVM versions
are quite demanding on machine resources and take long time
to build. Those who build their own packages can mitigate
this by setting LLVM_VERSION to some lower, saner value e.g.
11, then enabling the option would be less costly.