Fixes problems with line merging in the face of preprocessor directives.

This patch prepares being able to test for and fix more problems (see
FIXME in the test for example).

Previously we would output unwrapped lines for preprocessor directives
at the point where we also parsed the hash token. Since often
projections only terminate (and thus output their own unwrapped line)
after peeking at the next token, this would lead to the formatter seeing
the preprocessor directives out-of-order (slightly earlier). To be able
to correctly identify lines to merge, the formatter needs a well-defined
order of unwrapped lines, which this patch introduces.