Handle empty struct defs properly

[nrc]

We either don't reformat (struct structs) or lose comments (tuple structs). We should do neither.

e.g.,

struct Foo( /* a comment */ );
struct Foo();
struct Foo{ /* a comment */ }
struct Foo{}

should be idempotent

[tobarja]

Are you trying to say that those should remain untouched, but that the examples below are also idempotent, or that the code below should be reformatted to that above?

struct Foo ( /* a comment */ );
struct Foo ();
struct Foo { /* a comment */ }
struct Foo {}

[nrc]

I'm saying that the above should remain the same and the below should be changed to the above.

Although I actually made a mistake, there should be spaces before the opening braces:

struct Foo( /* a comment */ );
struct Foo();
struct Foo { /* a comment */ }
struct Foo {}