[llvm] [NFC] [MSAN] replace (void) with [[maybe_unused]] (PR #146617)

[Thurston Dang via llvm-commits]

================
@@ -5623,7 +5620,6 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
                             getOriginPtrForArgument(IRB, ArgOffset));
           }
         }
-        (void)Store;
----------------
thurstond wrote:

In release builds:
```
llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp:5566:16: error: variable 'Store' set but not used [-Werror,-Wunused-but-set-variable]
 5566 |         Value *Store = nullptr;
```


https://github.com/llvm/llvm-project/pull/146617