Add support for the HLSL "cast from zero" idiom (#1008)

If the user writes code like this:

    MyStruct s = (MyStruct) 0;

then we will interpret it as if they had written:

    MyStruct s = {};

That is, the "cast from zero" idiom will be taken as a legacy syntax for default construction (using an empty initializer list). This will be semantically equivalent to zero-initialization for all existing HLSL code (where `struct` fields can't have default initialization expressions defined), and is the easiest option for us to support in Slang (since we already support default-initialization using empty initializer lists).

The implementation of this feature is narrowly scoped:

* It only targets explicit cast expressions like `(MyStruct) 0` and not "constructor" syntax like `MyStruct(0)`
* It only applies when there is a single argument that is exactly an integer literal with a zero value (not a reference to a `static const int` that happens to be zero).

This change adds a test case to make sure that the feature works as expected. Because it relies on our existing initializer-list handling, the "cast from zero" idiom should work for any user-defined type where an initializer list would work.

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