AK: Resolve FIXME in Windows implementation of demangle()

[tomaszstrejczek]

This implements demangle() using Windows API. Also some rudimentary test is provided.

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[R-Goc]

Maybe we can pull in llvm demangle? It is fully self contained, and uses only std, not the llvm types.

[ADKaster]

Maybe we can pull in llvm demangle

Certainly not. Like I said, we only use this for debugging purposes. Performance of demangling is not a concern. Memory usage is not a concern. The thing that matters is not affecting normal release builds where we never hit this code path.

[R-Goc]

Didn't even think about perf here. It's header only and allocates its buffer. Seems like the sanest api to use. If not then we should make the buffer big. Like 4k or similar.