unused: assume all fields for a struct marked with structs.HostLayout are used

[cespare]

This is related to #355.

I have some...extremely unsafe code that looks like this:

// A closure is the internal representation of a Go func.
// (Actually a Go func is a pointer to one of these.)
type closure struct {
	_ structs.HostLayout

	code unsafe.Pointer
	//lint:ignore U1000 Included to mirror representation of Go func.
	ctx unsafe.Pointer
}

The reason I need the //lint:ignore is because I don't do a conversion using unsafe.Pointer(*closure), it is unsafe.Pointer(**closure), so I assume it's bypassing the logic you added for #355.

Anyway, I just added the structs.HostLayout hint (new in Go 1.23) as a way to formally document that I care about the memory layout of this struct. That seems like it could be a good hint to staticcheck that all the fields are needed, so it should skip the U1000 checks. (In fact, this seems like a nicer -- more explicit and more straightforward -- way of detecting "struct fields needed for unsafe reasons" going forward than staticcheck's current heuristics.)