Fix regression from r190382.

Make sure we perform the correct "referenced-but-not-used" check for
static member constants.

Fixes bug reported on cfe-commits by Alexey Samsonov.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190437 91177308-0d34-0410-b5e6-96231b3b80d8