[Carbon/C++ interop] Add support for C++ type short

[ivanaivanovska]

Added support for short/int16_t. Both function parameters and return values this type will be supported.

Demo:

// hello_short.h

#include <cstdint>
auto foo_short(int16_t a) -> int16_t;

// hello_short.cpp

#include "hello_short.h"
#include <cstdio>

auto foo_short(int16_t a) -> int16_t {
    printf("a = %i \n", a);
    return a;
}

// main.carbon

library "Main";

import Cpp library "hello_short.h";
import Core library "io";

fn Run() -> i32 {
  var a: i16 = 3;
  Cpp.foo_short(a);
  return 0;
}

$ clang -c hello_short.cpp
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link hello_short.o main.o --output=demo
$ ./demo
a = 3 

Part of #5263

[danakj]

LGTM