[llvm-cov] Remove workaround in line execution count calculation (PR34962)

Gap areas make it possible to correctly determine when to use counts
from deferred regions. Before gap areas were introduced, llvm-cov needed
to use a heuristic to do this: it ignored counts from segments that
start, but do not end, on a line. This heuristic breaks down on a simple
example (see PR34962).

This patch removes the heuristic and picks counts from any region entry
segment which isn't a gap area.

llvm-svn: 315960