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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217746 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed