Instead of passing a string that encodes the optional source-to-source
transformations to perform, just pass one optional, named argument for
each transformation.  It is clearer and easier to extend this way.

This ensures that normalized switch statements remain normalized.
However, if the label and the labeled statement are on different lines,
add an extra line directive corresponding to the label before the labeled
statement.

- New elaboration pass: SwitchNorm
    - recognizes structured 'switch' statements and puts them in a
      normalized form;
    - if selected, transforms unstructured 'switch' statements into a
      structured switch with goto actions + the original switch body
      with appropriate labels and gotos.

- C2C treatment of 'switch' statements is simplified accordingly.

- New language support option `-funstructured-switch`.

- Some tests were added (test/regression/switch3.c).

…on, `-finput-charset` and other small fixes)

This can occur in debug mode if there are declarations before the
first case, as in
```
  switch (x) {
    int x;
   case 0:
    ...
  }
```
Without this commit, the code above is a structured switch if -g is not given,
and a non-structured switch if -g is given.

Co-authored-by: Michael Schmidt <github@mschmidt.me>