unused: marking all fields as read doesn't work recursively

[dominikh]

Some operations mark all fields in a struct as used, because we can't do any better. One example is converting to unsafe.Pointer – who knows what the user will do with the data afterwards.

Unfortunately, we're only marking the immediate fields as used:

type t struct {
	a, b int
	c    t2
}

type t2 struct {
	d, e int
}

func Foo() {
	var x t
	_ = unsafe.Pointer(&x)
}

$ run-dev-staticcheck baz.go
baz.go:11:2: field d is unused (U1000)
baz.go:11:5: field e is unused (U1000)

Fields d and e should also be marked as used, as they are part of t's memory layout. The same problem occurs for embedded fields.