Fix use-after-free bug caught by address sanitizer:

http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-bootstrap/builds/2959

The location string is returned as a std::string, not a StringRef.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206032 91177308-0d34-0410-b5e6-96231b3b80d8