Instead of reaching into the IFC of the LineBuilder from the BFC, we
should let LineBuilder determine how to deal with the running vertical
float clearance. No functional changes.
If a block with inline children ends with a line break clearing any
floats, we not only need to take the introduced clearance into account
for the next line box, but the containing block needs to set the correct
height as well.
Since the spec calls for using the last line box' bottom as the resolved
height (if treated as auto), we now correctly apply the clearance to the
previous line box' bottom coordinate.
Fixes#4058.
