C++ Interop: Add support for char (#5988)

Added tests for different character types.
`char` is currently not in primitives prelude, so had to use full
prelude.

C++ Interop Demo:

```carbon
// main.carbon

library "Main";

import Cpp inline '''
auto output_char(char c) -> void {
  printf("%c", c);
}
''';

fn Run() -> i32 {
  let msg: array(Core.Char, 13) =
      ('H', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', '!', '\n');
  for (c: Core.Char in msg) {
    Cpp.output_char(c);
  }
  return 0;
}
```

```shell
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link main.o --output=demo
$ ./demo
Hello world!
```

Part of https://github.com/carbon-language/carbon-lang/issues/5263.

toolchain/check/testdata/interop/cpp/class/field.carbon

@@ -2,7 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/primitives.carbon
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/full.carbon
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run: