[llvm-cov] Unify region marker placement between text/html modes

Make sure that the text and html emitters always emit the same set of
region markers, and avoid emitting redundant markers for line segments
which don't end on the line they start on.

This is related to D35925, and depends on D36014

Differential Revision: https://reviews.llvm.org/D36020

llvm-svn: 312813