transpile: Translate enums in switch as their own type instead of integers

[Rua]

• Depends on transpile: Translate enums with newtype structs #1620
• Depends on transpile: Split off can_propagate_cast function, elide some more casts #1770
• Depends on transpile: Improve case translation #1776
• Fixes case statements do not preserve enum constants #1775

Enum constant names are now preserved in match arms where present, which significantly improves code fidelity.

[ahomescu]

This needs a rebase, but CI should pass now.

[ahomescu]

I think there might be one codegen bug here: for this C code

enum Bar { BarN1 = -1, Bar0, Bar1 };   // integral type: int
void f(int x) { enum Bar b = x; }

the transpiled Rust code fails to compile. Let me know if you reproduce it.

[Rua]

Yeah I could reproduce it. Fixed now, I hope.

[ahomescu]

This needs another rebase.

[ahomescu]

Needs rebase.

[fw-immunant]

A few small nits.

The first is commented inline on transpile: Add expected type for enums in `SwitchCases`­, and the other is that I'm not a big fan of transpile: Make `target_type_id` parameter of `convert_expr_with_cast` optional: it adds a branch to the function that makes the function not do what it says on the tin, which isn't what any of the existing callers want. Where transpile: Translate enums in `switch` as their own type instead of integers changes calls from convert_expr to the new convert_expr_with_cast, I'd prefer if it changed them to a new function (e.g. "convert_expr_possibly_casting" or such) that does that dispatch to either convert_expr or the old convert_expr_with_cast.

Otherwise, this LGTM and is a really clean idiomaticity win. Thanks for taking the numerous steps to make this improved translation happen!

This also seems to want a rebase now.

[Rua]

I've now added convert_expr_with_optional_cast instead.

[fw-immunant]

Looks great now, thanks!