[fir] Set !fir.len_param_index conversion to unimplemented

This patch is part of the upstreaming effort from fir-dev.

The conversion of len_param_index in fir-dev is incomplete, so for now
we're marking this as unimplemented until we can settle on a design for
the runtime support of LEN parameters.

Differential Revision: https://reviews.llvm.org/D114241



Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>