[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_and_lex_context.h

@@ -0,0 +1,40 @@
+// 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_DRIVER_H_
+#define EXECUTABLE_SEMANTICS_SYNTAX_DRIVER_H_
+
+#include <variant>
+
+#include "executable_semantics/ast/abstract_syntax_tree.h"
+#include "executable_semantics/syntax/parser.h"  // from parser.ypp
+
+namespace Carbon {
+
+// The state and functionality that is threaded "globally" through the
+// lexing/parsing process.
+class ParseAndLexContext {
+ public:
+  // Creates an instance analyzing the given input file.
+  ParseAndLexContext(const std::string& inputFile) : inputFileName(inputFile) {}
+
+  // Writes a syntax error diagnostic, containing message, for the input file at
+  // the given line, to standard error.
+  auto PrintDiagnostic(const char* message, int lineNumber) -> void;
+
+ private:
+  // A path to the file processed, relative to the current working directory
+  // when *this is called.
+  const std::string inputFileName;
+};
+
+}  // namespace Carbon
+
+// Gives flex the yylex prototype we want.
+#define YY_DECL int yylex(Carbon::ParseAndLexContext& context)
+
+// Declares yylex for the parser's sake.
+YY_DECL;
+
+#endif  // EXECUTABLE_SYNTAX_DRIVER_H_