[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.cpp

@@ -0,0 +1,20 @@
+// 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_and_lex_context.h"
+
+#include <cstring>
+#include <iostream>
+
+#include "executable_semantics/tracing_flag.h"
+
+// Writes a syntax error diagnostic, containing message, for the input file at
+// the given line, to standard error.
+auto Carbon::ParseAndLexContext::PrintDiagnostic(const char* message,
+                                                 int line_num) -> void {
+  std::cerr << inputFileName << ":" << line_num << ": " << message << std::endl;
+  exit(-1);  // TODO: do we really want this here?  It makes the comment and the
+             // name a lie, and renders some of the other yyparse() result
+             // propagation code moot.
+}