llvm-cov: Handle missing source files as GCOV does

If the source files referenced by a gcno file are missing, gcov
outputs a coverage file where every line is simply /*EOF*/.  This also
occurs for lines in the coverage that are past the end of a file that
is found.

This change mimics gcov.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208149 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed