Update Where we Apply Deprecated Attributes in C++

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This PR updates the deprecation logic of slice2cpp. Currently, on main, we ignore all [deprecated] metadata except when applied to an interface, or an operation. And deprecating an interface is just a shorthand for deprecating all it's operations...

If an operation is deprecated, we will generate [[deprecated]] on it's mapped functions, both in the generated Prx proxy class and the servant skeleton.

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This PR implements the first half of #2085 by removing the [[deprecated]] from the dispatch side code.

But it also expands what [[deprecated]] can be applied to, since it's very limited in C++ (compared to Java and C# where it's supported on most things). Now we support [deprecated] on:

• enums (we generate a single [[deprecated]] on the mapped enum)
• constants (we generate a single [[deprecated]] on the mapped const)
• structs and exceptions (we generate a single [[deprecated]] on the mapped struct/exception class)
• classes (we generate two [[deprecated]], one on the class, and one on the using XXXPtr we generate for the class).
• data members (we generate a single [[deprecated]] on the mapped field)

The logic for proxy code is unchanged. And the behavior for deprecating an interface has also changed. Previously it meant: just deprecate all it's operations, now deprecating an interface causes the compiler to deprecate the mapped proxy class.

[InsertCreativityHere]

I've been told to merge this as-is for now. I'll look into improving the string generation in the future.

For what it's worth, this is a general problem with our code-gen. Not specific to this PR. So it's probably better to fix it for all the places at once.