Goal is to increase type safety, though more work needs to be done (see
added TODOs).
Note that, after this change, check handlers corresponding to deleted
parse node kinds will no longer compile.
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Co-authored-by: Chandler Carruth <chandlerc@gmail.com>
Split `node.h` into separate files for ID types (`id.h`) and for typed
nodes (`typed_nodes.h`). The per-node-kind data is now specified as part
of declaring the typed nodes, and is removed from the node kinds
x-macros, which now simply enumerate the node kinds.
Continuing along with #3070. Note this is just a file rename, with BUILD
edits; every file previously in semantics/ should show as moved (except
maybe BUILDs, which split).