[llvm-cov] Filter away source files that aren't in the coverage mapping

... so that they don't show up in the index. This came up because polly
contains a .git directory and some other unmapped input in its source
dir.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282282 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed