internal/asan: add new package

The internal/asan package contains helper functions for manually
instrumenting code for the address sanitizer. It reexports the asan
routines in runtime uncoditionally, making the functions a no-op if the
build flag "asan" is disabled.

For [reserved]

src/internal/asan/asan.go

@@ -0,0 +1,22 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build asan
+
+package asan
+
+import (
+	"runtime"
+	"unsafe"
+)
+
+const Enabled = true
+
+func Read(addr unsafe.Pointer, len int) {
+	runtime.ASanRead(addr, len)
+}
+
+func Write(addr unsafe.Pointer, len int) {
+	runtime.ASanWrite(addr, len)
+}

src/internal/asan/doc.go

@@ -0,0 +1,11 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package asan contains helper functions for manually instrumenting code for the address sanitizer.
+
+The runtime package intentionally exports these functions only in the asan build;
+this package exports them unconditionally but without the "asan" build tag they are no-ops.
+*/
+package asan

src/internal/asan/noasan.go

@@ -0,0 +1,19 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !asan
+
+package asan
+
+import (
+	"unsafe"
+)
+
+const Enabled = false
+
+func Read(addr unsafe.Pointer, len int) {
+}
+
+func Write(addr unsafe.Pointer, len int) {
+}