Editor code actions generate failwith("TODO"), which no longer exists #9102

Found by a bug hunt over the 16 commits from eb087382 to cd641cbc.

#9052 removed failwith from Pervasives. The exhaustive-switch and add-missing-cases code actions still fill new branches with failwith("TODO"), and that code now fails to compile:

let f = (x: option<int>) => switch x { | None => failwith("TODO") | Some(_) => 1 }

The value failwith can't be found

At b123b7c3 the same code compiles with a deprecation warning.

The placeholder is built by Codegen.mk_fail_with_exp, which is used from type_utils.ml and codemod.ml. The analysis snapshots still expect it: tests/analysis_tests/tests/src/expected/ExhaustiveSwitch.res.txt and Codemod.res.txt.

The changelog contradicts itself on the same point. changes/deprecated-let-without-interface.fix.md says failwith and invalid_arg now emit their deprecation warning, while changes/remove-v12-deprecated-stdlib.breaking.md removes them.

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