[clangd] Extract Function: add hoisting support

[5chmidti]

Summary

Adds support to hoist variables declared inside the selected region
and used afterwards back out of the extraced function for later use.
Uses the explicit variable type if only one decl needs hoisting,
otherwise pair or tuple with auto return type deduction
(requires c++14) and a structured binding (requires c++17) or
explicitly unpacking the variables with get<>.

[5chmidti]

Fixup: rm added includes

[5chmidti]

Fix windows build by setting the c++ standard for the hoisting tests explicitly to c++17.

[5chmidti]

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[5chmidti]

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