Fix call unwrapping optimization

[AlexGuteniev]

Fix regression in #5808.

We can't move out lvalue reference.

The optimization for lvalue references is still possible, but we have to handle this case slightly differently.
Now, if the function is passed as lvalue, we copy from the passed function. instead of moving it out.

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Other cases of lvalue reference in call unwrapping:

• Inner move_only_function: [func.wrap.move.ctor]/6 and the definition of move_only_function enforces that attempt to use lvalue reference here won't compile. So no handling is needed.
• Inner copyable_function: will need to handle lvalue reference with this approach applied in one more place, in addition to these three places
• Inner function_ref. There can be lvalue references. But I don't know if we want to nullify moved-out function_ref or not. More natural would be to leave moved out function_ref intact, so we will always do as if we have lvalue reference.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for noticing and fixing this! 🏠 💥 🏚️ 🛠️ 🏡