[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.h

@@ -0,0 +1,25 @@
+// 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
+
+#ifndef EXECUTABLE_SEMANTICS_SYNTAX_PARSE_H_
+#define EXECUTABLE_SEMANTICS_SYNTAX_PARSE_H_
+
+#include <string>
+#include <variant>
+
+#include "executable_semantics/ast/abstract_syntax_tree.h"
+
+namespace Carbon {
+
+// This is the code given us by Bison, for now.
+using SyntaxErrorCode = int;
+
+// Returns the AST representing the contents of the named file, or an error code
+// if parsing fails.
+auto parse(const std::string& inputFileName)
+    -> std::variant<Carbon::AST, SyntaxErrorCode>;
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_SYNTAX_PARSE_H_