Fix unused result from sign extending an Offset.

Summary:
Offset was doubled in size, but the assignment was missing. We just need
to reassign to the original variable in this case to fix it.

Reviewers: cfe-commits, echristo

Subscribers: meikeb

Differential Revision: https://reviews.llvm.org/D24648

llvm-svn: 281706
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index deb3860..1e17afc0 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -3882,7 +3882,7 @@
   // possible.
   if (Ov) {
     assert(BitWidth <= UINT_MAX / 2 && "index (intermediate) result too big");
-    Offset.sext(2 * BitWidth);
+    Offset = Offset.sext(2 * BitWidth);
     sumOffsets(Offset, Addend, BinOpKind, AddendIsRight);
     return;
   }