Support basic int printing (#1405)

We don't have overload resolution, so I've simply added PrintInt for now. But it's really tempting to turn __intrinsic_print into a total kludge function.

explorer/testdata/basic_syntax/print.carbon

@@ -13,7 +13,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  var s: auto = "Hello world!\n";
+  var s: auto = "Hello world!";
   Print(s);
   return 0;
 }

explorer/testdata/basic_syntax/print_i32.carbon

@@ -0,0 +1,18 @@
+// 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
+//
+// RUN: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: Printing 1
+// CHECK: result: 0
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  PrintInt("Printing {0}", 1);
+  return 0;
+}