Handle invalid chars better, and do small cleanups around error output. (#657)

This adds a test for invalid characters (that would've failed before, because the printed char isn't escaped). Not sure if there's a good way to test the PrintDiagnostic code, as it appears to occur on bison parser errors, which I'm just not sure how to trigger.

executable_semantics/syntax/parser.ypp

@@ -61,7 +61,7 @@
 
 #include "executable_semantics/syntax/syntax_helpers.h"
 #include "executable_semantics/syntax/parse_and_lex_context.h"
-}
+}  // %code top
 
 %code requires {
 #include <optional>
@@ -73,21 +73,19 @@
 
 namespace Carbon {
 class ParseAndLexContext;
-}
+}  // namespace Carbon
 
-}
+}  // %code requires
 
 %code {
 
 extern int yylineno;
 
-void yy::parser::error(
-    const location_type&, const std::string& message)
-{
+void yy::parser::error(const location_type&, const std::string& message) {
   context.PrintDiagnostic(message, yylineno);
 }
 
-}
+}  // %code
 
 %token <int> integer_literal
 %token <char*> identifier