The lexer needs to process all #if[n]defs, even those found when processing is
disabled by a preceding failed conditional, or the first #endif in something
like:
#ifdef <undefined>
#ifdef whatever
#endif
#endif
terminates the first #ifdef and the second causes an error. And then the
processing of the inner #ifdefs needs to ignore their argument when they are
disabled by an outer failed condition.
Change-Id: Iba259498f1e16042f5b7580b9c000bb0599733d0
Reviewed-on: https://go-review.googlesource.com/14253 Reviewed-by: Rob Pike <r@golang.org>