cf: fully structurize functions that never return, as well.

[eddyb]

Note: this PR is a draft to avoid accidental merging onto its "base" branch (used as a form of ad-hoc PR stacking), and will remain as such, until its "base" branch can be set to main, i.e. all prerequisite PRs will have landed, up to and including this PR (whose branch is the "base" of this one):

• Add ConstKind::Undef (i.e. SPIR-V OpUndef). #37

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This was a small oversight - if a function never returned, structurization didn't even have the necessary type(s) to create undef constant(s) to fill the function body's outputs, so it left such functions partially unstructured.

The fix is mainly to pass the whole FuncDecl in, not just the FuncDefBody, giving access to the signature.