Lower varying parameters as pointers instead of SSA values.

[csyonghe]

This PR changes the way we lower entry point varying parameters so that they are emitted as a ConstRef<T> typed pointer value instead of a SSA value.

This is required to properly implement things like GetAttributeAtVertex or EvaluateAttributeAtCentroid, where the argument is expected to be a reference to a vertex attribute declaration.

Previously this is implemented as a hack assuming the x in GetAttributeAtVertex(x) is a load(attribute_var), which isn't safe or correct.

By lowering all varying parameters as pointers, we can properly implement GetAttributeAtVertex(x) assuming x is a pointer in the Input address space.

This also allowed us to support using types that needs storage lowering for vertex attributes, such as column_major matrix types or bool types, by reusing the existing lowerBufferElementTypeToStorageType pass, which requires any such type to be used via a buffer or a pointer. This lowering is required to support Optional<T> in vertex attributes.