Express prelude using Carbon syntax (#955)

This should be clearer, more maintainable, and more scalable than building the prelude AST by hand.

executable_semantics/main.cpp

@@ -5,12 +5,47 @@
 #include <cstdio>
 #include <cstring>
 #include <iostream>
+#include <string>
+#include <vector>
 
+#include "executable_semantics/common/arena.h"
+#include "executable_semantics/common/nonnull.h"
 #include "executable_semantics/interpreter/exec_program.h"
 #include "executable_semantics/syntax/parse.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/InitLLVM.h"
 
+// The Carbon prelude.
+//
+// TODO: Make this a separate source file that's embedded in the interpreter
+// at build time. See https://github.com/bazelbuild/rules_cc/issues/41 for a
+// possible mechanism.
+static constexpr std::string_view Prelude = R"(
+package Carbon api;
+
+// Note that Print is experimental, and not part of an accepted proposal, but
+// is included here for printing state in tests.
+fn Print(format_str: String) {
+  __intrinsic_print(format_str);
+}
+)";
+
+// Adds the Carbon prelude to `declarations`.
+static void AddPrelude(
+    Carbon::Nonnull<Carbon::Arena*> arena,
+    std::vector<Carbon::Nonnull<Carbon::Declaration*>>* declarations) {
+  std::variant<Carbon::AST, Carbon::SyntaxErrorCode> parse_result =
+      ParseFromString(arena, "<prelude>", Prelude, false);
+  if (std::holds_alternative<Carbon::SyntaxErrorCode>(parse_result)) {
+    // Try again with tracing, to help diagnose the problem.
+    ParseFromString(arena, "<prelude>", Prelude, true);
+    FATAL() << "Failed to parse prelude.";
+  }
+  const auto& prelude = std::get<Carbon::AST>(parse_result);
+  declarations->insert(declarations->begin(), prelude.declarations.begin(),
+                       prelude.declarations.end());
+}
+
 auto main(int argc, char* argv[]) -> int {
   llvm::setBugReportMsg(
       "Please report issues to "
@@ -38,6 +73,9 @@ auto main(int argc, char* argv[]) -> int {
     // Diagnostic already reported to std::cerr; this is just a return code.
     return *error;
   }
+  auto& ast = std::get<Carbon::AST>(ast_or_error);
+
+  AddPrelude(&arena, &ast.declarations);
 
   // Typecheck and run the parsed program.
   Carbon::ExecProgram(&arena, std::get<Carbon::AST>(ast_or_error),