[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/parser.ypp

@@ -11,33 +11,46 @@
 #include <list>
 
 #include "executable_semantics/syntax/syntax_helpers.h"
+#include "executable_semantics/syntax/parse_and_lex_context.h"
 }
 
 %code requires {
 #include <optional>
 
+#include "executable_semantics/ast/abstract_syntax_tree.h"
 #include "executable_semantics/ast/declaration.h"
 #include "executable_semantics/ast/field_list.h"
 #include "executable_semantics/ast/function_definition.h"
 
 namespace Carbon {
-// A representation of the parsed program.
-using AST = std::list<Carbon::Declaration>*;
+class ParseAndLexContext;
 }
 
 }
 
 %code {
+
 extern int yylineno;
-extern int yylex();
 
-void yyerror(std::optional<Carbon::AST> const& parsedProgram, char* error) {
-  Carbon::PrintSyntaxError(error, yylineno);
+void yyerror(
+    std::optional<Carbon::AST> const& /* parsedProgram */,
+    Carbon::ParseAndLexContext& context,
+    char* message
+) {
+  context.PrintDiagnostic(message, yylineno);
 }
 }
 
+//
+// Parameters to yyparse and yylex.
+//
+
+// "out" parameter passed to the parser, where the AST is written.
 %parse-param {std::optional<Carbon::AST>& parsedProgram}
 
+// "inout" parameter passed to both the parser and the lexer.
+%param {Carbon::ParseAndLexContext& context}
+
 %union {
    char* str;
    int num;