windows: use UTF16FromString and UTF16ToString from syscall

This CL updates the windows.UTF16FromString and windows.UTF16ToString
implementation so they forward to their syscall counterparts.

The standard library uses these particular syscall functions across
the board as it can't depend on x/sys. This means that even if the
syscall API is frozen, the implementation still receives improvements
and bug fixes, such as CL 425054.

Porting CL 425054 to x/sys is possible, but for this particular case
I think it's better to just call the syscall package. This way both
repos will benefit from future improvements (which I plan to do soon).

Change-Id: I305c4409142d79a57008d7e6d0717fb8986fe906
Reviewed-on: https://go-review.googlesource.com/c/sys/+/462875
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

windows/syscall_windows.go

@@ -10,7 +10,6 @@ import (
 	errorspkg "errors"
 	"fmt"
 	"runtime"
-	"strings"
 	"sync"
 	"syscall"
 	"time"
@@ -87,22 +86,13 @@ func StringToUTF16(s string) []uint16 {
 // s, with a terminating NUL added. If s contains a NUL byte at any
 // location, it returns (nil, syscall.EINVAL).
 func UTF16FromString(s string) ([]uint16, error) {
-	if strings.IndexByte(s, 0) != -1 {
-		return nil, syscall.EINVAL
-	}
-	return utf16.Encode([]rune(s + "\x00")), nil
+	return syscall.UTF16FromString(s)
 }
 
 // UTF16ToString returns the UTF-8 encoding of the UTF-16 sequence s,
 // with a terminating NUL and any bytes after the NUL removed.
 func UTF16ToString(s []uint16) string {
-	for i, v := range s {
-		if v == 0 {
-			s = s[:i]
-			break
-		}
-	}
-	return string(utf16.Decode(s))
+	return syscall.UTF16ToString(s)
 }
 
 // StringToUTF16Ptr is deprecated. Use UTF16PtrFromString instead.