Add a basic Core.Print function for ints. (#4078)

We'd been discussing that explorer remains necessary for print, and I
was wondering if this kind of approach would be okay (we _probably_ want
this to work, based on #2110, albeit with more overloads -- but I don't
think there's a good way to support overloads at the moment).

```
╚╡../bazel-bin/examples/sieve
2
3
5
7
11
13
17
19
23
29
31
37
41
43
...
```

toolchain/check/eval.cpp

@@ -10,6 +10,7 @@
 #include "toolchain/sem_ir/builtin_function_kind.h"
 #include "toolchain/sem_ir/function.h"
 #include "toolchain/sem_ir/ids.h"
+#include "toolchain/sem_ir/inst_kind.h"
 #include "toolchain/sem_ir/typed_insts.h"
 
 namespace Carbon::Check {
@@ -709,6 +710,11 @@ static auto MakeConstantForBuiltinCall(Context& context, SemIRLoc loc,
     case SemIR::BuiltinFunctionKind::None:
       CARBON_FATAL() << "Not a builtin function.";
 
+    case SemIR::BuiltinFunctionKind::PrintInt: {
+      // Providing a constant result would allow eliding the function call.
+      return SemIR::ConstantId::NotConstant;
+    }
+
     case SemIR::BuiltinFunctionKind::IntMakeType32: {
       return context.constant_values().Get(SemIR::InstId::BuiltinIntType);
     }