Fix moc preprocessor-only mode with input that contains seemingly invalid identifiers

Fix moc preprocessor-only mode with input that contains seemingly invalid identifiers

In WebKit we use moc -E to pre-process various files before throwing at
further build creation tools. The pre-processing is used to filter out
code depending in #ifdef'fed features.

The latest addition to the family of pre-processed files is the CSS grammar,
which is written in Bison. It contains rule lines like

   $$ = parser->createFoo()

and when pre-processing this moc stumbles over the dollar sign. Instead
of ignoring un-tokenizable input we should add it to the current token
if we're in preprocessor-only mode, otherwise the $$ gets eaten and we
produce data-loss by printing out less characters than.

Change-Id: Ib32e7c04b38dd2ba3726201e76f27405f7ea6c0d
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>