internal/syscall/windows: change WSAMsg.Name type

The problem was discovered while running

go test -a -short -gcflags=all=-d=checkptr -run=TestUDPConnSpecificMethods net

WSAMsg is type defined by Windows. And WSAMsg.Name could point to two
different structures for IPv4 and IPV6 sockets.

Currently WSAMsg.Name is declared as *syscall.RawSockaddrAny. But that
violates

(1) Conversion of a *T1 to Pointer to *T2.

rule of

https://golang.org/pkg/unsafe/#Pointer

When we convert *syscall.RawSockaddrInet4 into *syscall.RawSockaddrAny,
syscall.RawSockaddrInet4 and syscall.RawSockaddrAny do not share an
equivalent memory layout.

Same for *syscall.SockaddrInet6 into *syscall.RawSockaddrAny.

This CL changes WSAMsg.Name type to *syscall.Pointer. syscall.Pointer
length is 0, and that at least makes type checker happy.

After this change I was able to run

go test -a -short -gcflags=all=-d=checkptr std cmd

without type checker complaining.

Updates #34972

Change-Id: Ic5c2321c20abd805c687ee16ef6f643a2f8cd93f
Reviewed-on: https://go-review.googlesource.com/c/go/+/222457


Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>