Cleanup 'addImport' and Surrounding Functions in slice2swift

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In slice2swift we have two versions of getTopLevelModule and addImport.
One takes a ContainedPtr and one takes a TypePtr.

But, the versions which took TypePtr were weird and not really necessary.
They basically just converted the TypePtr into a ContainedPtr to call the other one.
But most of the places where it was called, the Type was already a Contained.

Anyways, I removed the TypePtr versions.
And because of this, alot of unnecessary dynamic_pointer_casts.

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It's fine to pass a nullptr to addImport.
So checking base before calling it is unnecessary.

Also no need to cast to Contained now. ClassDef already inherits from Contained.

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Instead of having 2 addImport which do the same thing,
now we only have 1 which takes the common base class of Type and Contained: SyntaxTreeBase.

[externl]

The original code in 3.7 made sense.

ice/cpp/src/slice2swift/SwiftUtil.cpp

Lines 213 to 221 in d53f0a9

	ModulePtr
	Slice::getTopLevelModule(const TypePtr& type)
	{
	assert(ProxyPtr::dynamicCast(type) || ContainedPtr::dynamicCast(type));
	ProxyPtr proxy = ProxyPtr::dynamicCast(type);
	return getTopLevelModule(proxy ? ContainedPtr::dynamicCast(proxy->_class()) :
	ContainedPtr::dynamicCast(type));
	}

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That makes sense since back then we didn't have an Interface type in the compiler. Just Class and Proxy.
So, when all this got shuffled around, I guess that this function was mechanistically updated into a useless form.

[externl]

Looks good to me!

[externl]

Why don't we need to check ret->definitionContext() anymore?

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Because:
dynamic_pointer_cast<Contained>(p) == true
implies that
p->definitionContext() != null

Both of these are about checking the same thing: "is this a user-defined type?"

Because any user defined check will have a definitionContext (as opposed to something like int, a built-in type).
And will also be a subclass of Contained in the compiler.