[llvm-cov] Get rid of all invalid filename references

We used to append filenames into a vector of std::string, and then
append a reference to each string into a separate vector. This made it
easier to work with the getUniqueSourceFiles API. But it's buggy.

std::string has a small-string optimization, so you can't expect to
capture a reference to one if you're copying it into a growing vector.
Add a test that triggers this invalid reference to std::string scenario,
and kill the issue with fire by just using ArrayRef<std::string>
everywhere.

llvm-svn: 282281