[LTO] Fix a use-after-free introduced in r278907 and caught by ASan.

The ASan build bot caught this right away:
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast/builds/15580/steps/check-llvm%20asan/logs/stdio

This was also breaking a Windows build bot I'm pretty sure.

llvm-svn: 278912