[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