[Coverage] Attempt to appease a Windows builder

The builder prints out the following IR:

  \5CCoverageMapping\5COutput\5Ctest\5Cf1.c

The updated test in r276367 expects path separators to be either '/' or
'\\', so it chokes on the unexpected "5C" stuff. I'm not sure what that
is, but I included a kludge that should work around it.

Failing bot:

  http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast/builds/8718

llvm-svn: 276370