[executable semantics] Add Syntax driver (#362)

Slowly bringing this into line with Bison's C++ example parser
so we can use strong semantic values for symbols rather than
leaking pointers.  First step is to thread a `ParseAndLexContext` 
object through the whole syntactic analysis state, like the 
example has.  In the example, it's called `driver`.

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/syntax/parse.cpp

@@ -0,0 +1,44 @@
+// 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
+
+#include "executable_semantics/syntax/parse.h"
+
+#include <iostream>
+
+#include "executable_semantics/syntax/parse_and_lex_context.h"
+#include "executable_semantics/tracing_flag.h"
+
+extern FILE* yyin;
+
+namespace Carbon {
+
+// Returns an abstract representation of the program contained in the
+// well-formed input file, or if the file was malformed, a description of the
+// problem.
+auto parse(const std::string& inputFileName)
+    -> std::variant<AST, SyntaxErrorCode> {
+  yyin = fopen(inputFileName.c_str(), "r");
+  if (yyin == nullptr) {
+    std::cerr << "Error opening '" << inputFileName
+              << "': " << std::strerror(errno) << std::endl;
+    exit(1);
+  }
+
+  std::optional<AST> parsedInput = std::nullopt;
+  ParseAndLexContext context(inputFileName);
+
+  auto syntaxErrorCode = yyparse(parsedInput, context);
+  if (syntaxErrorCode != 0) {
+    return syntaxErrorCode;
+  }
+
+  if (parsedInput == std::nullopt) {
+    std::cerr << "Internal error: parser validated syntax yet didn't produce "
+                 "an AST.\n";
+    exit(1);
+  }
+  return *parsedInput;
+}
+
+}  // namespace Carbon