[llvm-cov] Attempt to appease an older builder

It's using a version of clang which can't (or won't) deduce an implicit
conversion from a SmallString to a StringRef. Write the conversion out
explicitly:

  http://lab.llvm.org:8011/builders/lldb-x86_64-ubuntu-14.04-buildserver/builds/8574

llvm-svn: 275647