llvm-cov: Generalize -filename-equivalence

The filename-equivalence flag allows you to show coverage when your
source files don't have the same full paths as those that generated
the data. This is mostly useful for writing tests in a cross-platform
way.

This wasn't triggering in cases where the filename was derived
directly from the coverage data, which meant certain types of test
case were impossible to write. This patch fixes that, and following
patches involve tests that need this.
llvm-cov: Fix dropped lines when filters were applied

Uncovered lines in the middle of a covered region weren't being shown
when filtering to a particular function.