llvm-cov: Simplify CounterMappingRegion, pushing logic to its user

A single function in SourceCoverageDataManager was the only user of
some of the comparisons in CounterMappingRegion, and at this point we
know that only one file is relevant. This lets us use slightly simpler
logic directly in the client.
llvm-cov: Clean up some redundancy in the view API (NFC)

This removes the need to pass a starting and ending line when creating
a SourceCoverageView, since these are easy to determine.