Hello world C++ interop example (#5991)

Based on #5920.
Added commented out better examples and clarified what is missing to
support them.
Added `tags` support to `carbon_binary` (based on #5967) to set the
`BUILD` rule to manual until we can find `cstdio` in macos.

```shell
$ bazel run examples/interop/cpp:hello_world
...
Hello world!
```

examples/interop/cpp/BUILD

@@ -0,0 +1,12 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+load("//bazel/carbon_rules:defs.bzl", "carbon_binary")
+
+carbon_binary(
+    name = "hello_world",
+    srcs = ["hello_world.carbon"],
+    # TODO: Remove when macos can find cstdio.
+    tags = ["manual"],
+)

examples/interop/cpp/hello_world.carbon

@@ -0,0 +1,33 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+import Cpp inline "#include <cstdio>";
+
+fn Run() {
+  // TODO: Requires operator <<, declaration type Var (cout) and class with
+  // virtual bases (basic_ostream).
+  // Cpp.std.cout << "Hello world!\n";
+
+  // TODO: Requires variadic function.
+  // Cpp.printf("Hello world!\n");
+
+  // TODO: Requires nullable pointer.
+  // Cpp.puts("Hello world!\n");
+
+  // TODO: Requires nullable void pointer.
+  // Cpp.write(1, "Hello world!\n", 13);
+
+  // TODO: Requires Core.String API.
+  // let message: str = "Hello world!\n\n";
+  // for (c: Core.Char in msg) {
+  //   Cpp.putchar((c as u8) as i32);
+  // }
+
+  let message: array(Core.Char, 13) =
+      ('H', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', '!', '\n');
+  for (c: Core.Char in message) {
+    // TODO: u8 should probably have an implicit cast to i32.
+    Cpp.putchar((c as u8) as i32);
+  }
+}