[ORC] Fix a warning about an unused variable on Windows. NFC.

Differential Revision: https://reviews.llvm.org/D131055

(cherry picked from commit 46196db)

Author: mstorsjo

llvm/lib/ExecutionEngine/Orc/TargetProcess/ExecutorSharedMemoryMapperService.cpp

@@ -241,6 +241,7 @@ Error ExecutorSharedMemoryMapperService::release(
                                            errno, std::generic_category())));
 
 #elif defined(_WIN32)
+    (void)Size;
 
     if (!UnmapViewOfFile(Base.toPtr<void *>()))
       Err = joinErrors(std::move(Err),