Suppress an annoying "unused variable" warning caused by bug 17897.

Clang says that "flow" is unused when building LLD. This patch suppresses it.

Differential Revision: http://llvm-reviews.chandlerc.com/D2573

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199922 91177308-0d34-0410-b5e6-96231b3b80d8