gen-guest-c: Emit prettier comments.

[sunfishcode]

Don't insert a newline between a type and its documentation comment. And do insert a newline before the documentation comment. This makes the generated header files prettier.

For example this:

// A timestamp in nanoseconds.
typedef uint64_t wasi_clocks_instant_t;

// A time and date in seconds plus nanoseconds.
typedef struct {
  uint64_t seconds;
  uint32_t nanoseconds;
} wasi_clocks_datetime_t;

Instead of this:

// A timestamp in nanoseconds.

typedef uint64_t wasi_clocks_instant_t;
// A time and date in seconds plus nanoseconds.

typedef struct {
  uint64_t seconds;
  uint32_t nanoseconds;
} wasi_clocks_datetime_t;