Make WindowsRuntimeDefaultInterface attribute trimmable

[Sergio0694]

Summary

Move the [WindowsRuntimeDefaultInterface] attribute from individual projected runtime class types to a centralized WindowsRuntimeDefaultInterfaces static class in the ABI namespace. This decouples the default interface Type references from the class types themselves, allowing the trimmer to remove unused interface types at publish time.

Previously, each projected runtime class carried a [WindowsRuntimeDefaultInterface(typeof(IDefaultInterface))] attribute directly on the class. Because attributes are not trimmable, this forced the linker to preserve the interface type even when it was never used at runtime. The attribute is only consumed by the interop generator (WinRT.Interop.Generator) to compute WinRT type signatures.

Changes

Attribute definition (WindowsRuntimeDefaultInterfaceAttribute)

• Changed from single Type parameter (interfaceType) to two Type parameters (runtimeClassType, interfaceType)
• Set AllowMultiple = true since the centralized type needs multiple attribute instances

Code generator (cswinrt.exe)

• Removed per-class [WindowsRuntimeDefaultInterface] emission from write_class
• Added add_default_interface_entry to accumulate (class, interface) pairs during parallel namespace processing
• Added write_default_interfaces_class to emit a single WindowsRuntimeDefaultInterfaces.cs file after all namespaces are processed, with entries sorted for deterministic output
• Guarded with !settings.reference_projection since the file is only needed for implementation assemblies
• Used write_begin() for the file header, consistent with all other generated files

Interop generator (WinRT.Interop.Generator)

• Updated TryGetDefaultInterfaceFromAttribute to look up the centralized ABI.WindowsRuntimeDefaultInterfaces type instead of searching for the attribute on each individual class
• Added GetDefaultInterfacesLookup extension method on ModuleDefinition following the same caching pattern as GetTopLevelTypesLookup: a ConditionalWeakTable holding a FrozenDictionary<(Namespace, Name), TypeSignature> for O(1) lookups
• Removed the InteropReferences parameter from TryGetDefaultInterfaceFromAttribute since attribute type validation is no longer needed (the centralized type only has these attributes)

Bug fix

• Fixed write_begin_interface_iids in type_writers.h which was hardcoding version 0.0.0-private.0 instead of using VERSION_STRING

Generated output (before → after)

Before (on each class):

[WindowsRuntimeDefaultInterface(typeof(IHttpMethod))]
public sealed class HttpMethod : WindowsRuntimeObject, ...

After (centralized file):

// WindowsRuntimeDefaultInterfaces.cs
namespace ABI
{
[WindowsRuntimeDefaultInterface(typeof(global::Windows.Web.Http.HttpMethod), typeof(global::ABI.Windows.Web.Http.IHttpMethod))]
// ... one attribute per runtime class ...
internal static class WindowsRuntimeDefaultInterfaces;
}