go/printer: deletes or inserts AST types into code

[stamblerre]

Forked from #31291.

We have found that go/printer.Fprint will delete broken code.
An example of code being deleted: https://play.golang.org/p/WKRt74denE0.

We have also found that go/printer.Fprint will insert the string literal BadStmt or BadExpr in broken code that parses into *ast.BadStmts or *ast.BadExprs.
An example of code being rewritten: https://play.golang.org/p/_BBrwbfAtEH.

This was discovered through using the function go/format.Node, which uses go/printer.Fprint. We noticed that format.Source returns an error in such cases, which is the behavior I would expect. I would have expected that format.Source and format.Node would behave the same on the same input.

[mvdan]

Does this happen with go/printer too, the underlying package? If so, the issue should probably be retitled.

[stamblerre]

Looks like it does, thanks! Updated the issue, title, and playground links to reflect this.