all: transfer reflect.{SliceHeader, StringHeader} to unsafeheader.{Slice, String}

After we deprecated reflect.{SliceHeader, StringHeader}, it is recommended
to use unsafe.{Slice, String} to replace its work. However, the compiler
and linker cannot be migrated for the time being.

As a temporary strategy, using the "internal/unsafeheader" package like
other code is the most suitable choice at present.

For #53003.

Change-Id: I69d0ef72e2d95caabd0706bbb247a719d225c758
Reviewed-on: https://go-review.googlesource.com/c/go/+/429755
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: hopehook <hopehook@golangcn.org>

src/cmd/compile/internal/base/mapfile_mmap.go

@@ -8,8 +8,8 @@
 package base
 
 import (
+	"internal/unsafeheader"
 	"os"
-	"reflect"
 	"runtime"
 	"syscall"
 	"unsafe"
@@ -34,10 +34,10 @@ func MapFile(f *os.File, offset, length int64) (string, error) {
 	}
 
 	buf = buf[x:]
-	pSlice := (*reflect.SliceHeader)(unsafe.Pointer(&buf))
+	pSlice := (*unsafeheader.Slice)(unsafe.Pointer(&buf))
 
 	var res string
-	pString := (*reflect.StringHeader)(unsafe.Pointer(&res))
+	pString := (*unsafeheader.String)(unsafe.Pointer(&res))
 
 	pString.Data = pSlice.Data
 	pString.Len = pSlice.Len

src/cmd/link/internal/ld/outbuf_windows.go

@@ -5,7 +5,7 @@
 package ld
 
 import (
-	"reflect"
+	"internal/unsafeheader"
 	"syscall"
 	"unsafe"
 )
@@ -35,8 +35,8 @@ func (out *OutBuf) Mmap(filesize uint64) error {
 	if err != nil {
 		return err
 	}
-	bufHdr := (*reflect.SliceHeader)(unsafe.Pointer(&out.buf))
-	bufHdr.Data = ptr
+	bufHdr := (*unsafeheader.Slice)(unsafe.Pointer(&out.buf))
+	bufHdr.Data = unsafe.Pointer(ptr)
 	bufHdr.Len = int(filesize)
 	bufHdr.Cap = int(filesize)
 

src/reflect/value.go

@@ -2155,7 +2155,7 @@ func (v Value) Pointer() uintptr {
 		return uintptr(p)
 
 	case Slice:
-		return (*SliceHeader)(v.ptr).Data
+		return uintptr((*unsafeheader.Slice)(v.ptr).Data)
 	}
 	panic(&ValueError{"reflect.Value.Pointer", v.kind()})
 }