Use equals_lower() rather than creating a temporary string with lower().

More important than any performance concerns, the code was dropping the
temporary string on the floor after assigning it to a StringRef, and
then used the StringRef later. Caught by running the LLD tests under
ASan.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@193486 91177308-0d34-0410-b5e6-96231b3b80d8