Hopefully avoid a warning on some GCCs (but not all strangely,
preventing me from seeing it initially). GCC doesn't use the unused
attribute on members for anything, so while it works to suppress Clang's
warning for an unused private member, it adds a GCC warning for the
attribute. =/ Silence Clang's warning with a void cast in the
constructor instead which doesn't trigger any complaints from GCC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206373 91177308-0d34-0410-b5e6-96231b3b80d8