Configure reentrant bison/flex (#789)

Make the parser reentrant, working towards #769 and the ability to load other packages cleanly.

executable_semantics/syntax/parse.h

@@ -17,7 +17,7 @@ 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& input_file_name)
+auto Parse(const std::string& input_file_name)
     -> std::variant<Carbon::AST, SyntaxErrorCode>;
 
 }  // namespace Carbon