Fix "set debug parser"

While debugging my longer series, I discovered that I broken "set
debug parser" a couple years ago.  This patch fixes it and adds a
minimal test case so that it, hopefully, will not break again.

This patch also adds parser debugging to the C++ name canonicalizer.

Thanks to Tom de Vries for fixing the test case.

gdb/parse.c

@@ -60,8 +60,8 @@ show_expressiondebug (struct ui_file *file, int from_tty,
 }
 
 
-/* True if an expression parser should set yydebug.  */
-static bool parser_debug;
+/* See parser-defs.h.  */
+bool parser_debug;
 
 static void
 show_parserdebug (struct ui_file *file, int from_tty,