Defeat a GCC -Wunused-result warning

It was warning like:

../llvm-project/llvm/lib/Support/ErrorHandling.cpp:172:51: warning:
ignoring return value of ‘ssize_t write(int, const void*, size_t)’,
         declared with attribute warn_unused_result [-Wunused-result]
            (void)::write(2, OOMMessage, strlen(OOMMessage));

Work around the warning by storing the return value in a variable and
casting that to void instead. We already did this for the other write()
call in this file.

llvm-svn: 308483