[ARM64] Fix a heap-use-after-free spotted by ASan.

StringRef::lower() returns a std::string. Better yet, we can now stop
thinking about what it returns and write 'auto'. It does the right
thing. =]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205135 91177308-0d34-0410-b5e6-96231b3b80d8