Fix use-of-temporary with StringRef in code coverage

The fixed code is basically identical to the same loop below, which
might indicate an opportunity for refactoring. I just wanted to fix
the use-of-temporary issue.

Caught by adding a similar check to StringRef as r283798 did for
ArrayRef. I'll be upstreaming that soon.

Reviewed by Vedant Kumar as https://reviews.llvm.org/D26317.

llvm-svn: 286122