Silence 'set but not used' warning when building in release mode using gcc.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184299 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaCast.cpp b/lib/Sema/SemaCast.cpp
index 3b01c22..dff06b7 100644
--- a/lib/Sema/SemaCast.cpp
+++ b/lib/Sema/SemaCast.cpp
@@ -759,6 +759,7 @@
     VirtualBase = VirtualBase && IsVirtual;
   }
 
+  (void) NonZeroOffset; // Silence set but not used warning.
   assert((VirtualBase || NonZeroOffset) &&
          "Should have returned if has non-virtual base with zero offset");