llvm-cov: Sink some reporting logic into CoverageMapping

This teaches CoverageMapping::getCoveredFunctions to filter to a
particular file and uses that to replace most of the logic found in
llvm-cov report.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221962 91177308-0d34-0410-b5e6-96231b3b80d8