Can let an any, its typ type metadata and data value do not match?

[qiu laidongfeng2]

In #21031, this can be optimized to reduce the Pointer,usage atomic-operate for any type, as shown in https://go-review.googlesource.com/c/go/+/617695 .

But we need support for setting atomic values to nil, and we also need a way to represent deleted values.

The current practice is to make a spin lock using the typ field of any.

I want to ask if we can do better this way:

typ remains unchanged after the first setting,Cases such as being deleted are represented by a special internal package pointer, which is set to data field.

An example to illustrate this approach:

```go

package main

import (

"sync/atomic"

"unsafe"

)

var delete byte

func main() {

var v atomic.Value

var i int = 1

v.Store(i)

// Make a any , it is typ=int meta data,data=&byte

nv := new_delete(getIntType())

// Indicates deleted

v.Store(nv)

}

type iface struct {

typ, data unsafe.Pointer

}

func new_delete(typ unsafe.Pointer) any {

ret := iface{

typ: typ,

data: unsafe.Pointer(&delete),

}

return *(*any)(unsafe.Pointer(&ret))

}

func getIntType() unsafe.Pointer {

var i int

var a any = i

var as *iface = (*iface)(unsafe.Pointer(&a))

return as.typ

}

```