Preproceessor: fix #if skipping under -traditional-cpp.

When parsing directives within skipped #if blocks, we don't want to retain
any whitespace. Previously we were just skipping comments, but it's not
possible to skip comments and retain other whitespace. This change matches
the usual behavior for parsing directives (i.e. the behavior outside of
skipped #if blocks).

<rdar://problem/13267695>