accept spmd command blocks as language elements (bug #58676) * pt-spmd.h, pt-spmd.cc: New files. * libinterp/parse-tree/module.mk: Update. * parse.h, oct-parse.yy (base_parser::make_spmd_command): New function. (spmd_command): New non-terminal. (command): Include spmd_command in list of possible commands. * pt-all.h: Also include pt-spmd.h. * pt-walk.h, pt-walk.cc (tree_walker::visit_spmd_command): New virtual function. * pt-eval.h, pt-eval.cc (tree_evaluator::visit_spmd_command): New function. * pt-pr-code.h, pt-pr-code.cc (tree_print_code::visit_spmd_command): New function.

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