Add prelude support for implicit conversion between integer literals and sized integer types. (#4525)

In preparation for changing integer literals to be of `IntLiteral` type.
Conversions from the integer literal type are only permitted when the
value fits within the destination type.

For now, if conversion cannot be checked because the source value is a
symbolic constant, produce a symbolic constant representing the
conversion rather than rejecting it.

toolchain/check/eval.cpp

@@ -1088,6 +1088,9 @@ static auto MakeConstantForBuiltinCall(Context& context, SemIRLoc loc,
 
     // Integer conversions.
     case SemIR::BuiltinFunctionKind::IntConvertChecked: {
+      if (phase == Phase::Symbolic) {
+        return MakeConstantResult(context, call, phase);
+      }
       return PerformCheckedIntConvert(context, loc, arg_ids[0], call.type_id);
     }