preprocessor: do not destroy double slash escaped identifiers

preprocessor: do not destroy double slash escaped identifiers

The preprocessor currently destroys double slash containing escaped
identifiers (for example \a//b ). This is due to next_token trying to
convert single line comments (//) into /* */ comments. This then leads
to an unintuitive error message like this:
ERROR: syntax error, unexpected '*'

This patch fixes the error by recognizing escaped identifiers and
returning them as single token. It also adds a testcase.