[llvm-cov] Move LineCoverageIterator to libCoverage. NFC.

LineCoverageIterator makes it easy for clients of coverage data to
determine line execution counts for a file or function. The coverage
iteration logic is tricky enough that it really pays not to have
multiple copies of it. Hopefully having just one implementation in LLVM
will make the iteration logic easier to test, reuse, and update.

This commit is NFC but I've added a unit test to go along with it just
because it's easy to do now.

llvm-svn: 316141