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/parse_and_lex_context.cpp

@@ -7,15 +7,14 @@
 #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 std::string& message,
                                                  int line_num) -> void {
+  // TODO: Do we really want this to be fatal?  It makes the comment and the
+  // name a lie, and renders some of the other yyparse() result propagation code
+  // moot.
   std::cerr << input_file_name << ":" << 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.
+  exit(-1);
 }