[ORC] Actually propagate memory unmapping errors on Windows

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),
            ^~~~~~~~~~ ~~~~~~~~~~~~~~~

Differential Revision: https://reviews.llvm.org/D131056

(cherry picked from commit 46bc1b5)

Author: mstorsjo

llvm/lib/ExecutionEngine/Orc/MemoryMapper.cpp

@@ -361,8 +361,8 @@ void SharedMemoryMapper::release(ArrayRef<ExecutorAddr> Bases,
 #elif defined(_WIN32)
 
       if (!UnmapViewOfFile(Reservations[Base].LocalAddr))
-        joinErrors(std::move(Err),
-                   errorCodeToError(mapWindowsError(GetLastError())));
+        Err = joinErrors(std::move(Err),
+                         errorCodeToError(mapWindowsError(GetLastError())));
 
 #endif