[ORC] Actually propagate memory unmapping errors on Windows

[mstorsjo]

Summary

This fixes warnings like these:

../lib/ExecutionEngine/Orc/MemoryMapper.cpp:364:9: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result]
        joinErrors(std::move(Err),
        ^~~~~~~~~~ ~~~~~~~~~~~~~~~

[argentite]

That was a mistake, Thanks.

llvm/lib/ExecutionEngine/Orc/MemoryMapper.cpp

#if defined(LLVM_ON_UNIX)
 
       if (munmap(Reservations[Base].LocalAddr, Reservations[Base].Size) != 0)
         Err = joinErrors(std::move(Err), errorCodeToError(std::error_code(
                                              errno, std::generic_category())));
 
 #elif defined(_WIN32)
 
       if (!UnmapViewOfFile(Reservations[Base].LocalAddr))
-        joinErrors(std::move(Err),
+        Err = joinErrors(std::move(Err),
                          errorCodeToError(mapWindowsError(GetLastError())));
 
 #endif
 
       Reservations.erase(Base);
     }
   }
 
   EPC.callSPSWrapperAsync<