[llc] Fix sanitizer failure.

Store the string on the stack rather than just the StringRef.

Fix sanitizer bots:

  http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast/builds/19948
  http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-bootstrap-msan/builds/5500

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335423 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed