Fix MarshalAs IID parameter index parsing

[Copilot]

Fixes native AOT outerloop runs that are currently broken.

NativeAOT could assert while parsing valid MarshalAs metadata emitted for COM interface parameters using IidParameterIndex, leaving unread bytes in the marshal descriptor blob.

• Parser

  • Consume optional IidParameterIndex payloads for COM interface native types:
    • IUnknown
    • IDispatch
    • Interface
  • Keep existing behavior of using only the native type in NativeAOT.

• Regression coverage

  • Added TypeSystem coverage for descriptors like:

[MarshalAs(UnmanagedType.Interface, IidParameterIndex = 0)] out object value

• Covered Interface, IUnknown, and IDispatch marshal descriptors.

[dotnet-policy-service]

Tagging subscribers to this area: @agocke, @dotnet/ilc-contrib
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[MichalStrehovsky]

/azp run runtime-nativeaot-outerloop

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Azure Pipelines successfully started running 1 pipeline(s).

[Copilot]

Pull request overview

This PR updates the TypeSystem marshal descriptor parser to tolerate (and consume) the optional IidParameterIndex payload that can appear for COM-interface native types, and adds TypeSystem regression coverage to ensure these marshal blobs no longer trigger debug assertions due to unread bytes.

Changes:

• Update EcmaSignatureParser.ParseMarshalAsDescriptor() to consume an optional compressed integer payload for IUnknown, IDispatch, and Interface native types.
• Add TypeSystem tests that compile methods using [MarshalAs(..., IidParameterIndex = 0)] and validate the descriptor parses successfully.
• Extend the CoreTestAssembly MarshalAsAttribute stub and add new methods that emit the relevant marshal descriptors.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/coreclr/tools/Common/TypeSystem/Ecma/EcmaSignatureParser.cs	Consumes optional IidParameterIndex data for COM interface marshal descriptors to avoid leftover bytes/asserts.
src/coreclr/tools/aot/ILCompiler.TypeSystem.Tests/MarshalUtilsTests.cs	Adds regression test validating the marshal descriptor type parses for COM interface cases with IidParameterIndex.
src/coreclr/tools/aot/ILCompiler.TypeSystem.Tests/CoreTestAssembly/Platform.cs	Updates the CoreTestAssembly MarshalAsAttribute stub to allow IidParameterIndex named argument in test code.
src/coreclr/tools/aot/ILCompiler.TypeSystem.Tests/CoreTestAssembly/Marshalling.cs	Adds test methods that use MarshalAs(..., IidParameterIndex = 0) for IUnknown/IDispatch/Interface.

[MichalStrehovsky]

/azp run runtime-nativeaot-outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

[MichalStrehovsky]

@dotnet/ilc-contrib could someone have a look? #128214 broke outerloop builds and we can't test

[MichalStrehovsky]

/ba-g helix issue