Implement NoSEH flag setting for crossgened modules in ILLink

[Copilot]

This PR implements the product fix to ensure the NoSEH flag is correctly set in the PE header when ILLink processes crossgened assemblies.

Fixes #117752

Changes Made

Product Fix:

• Updated WriteAssembly() method in OutputStep.cs to set module.Characteristics |= ModuleCharacteristics.NoSEH when processing crossgened modules
• The NoSEH flag is now set alongside existing IL-only attributes for crossgened assemblies

Technical Details

The implementation ensures that when a module is detected as crossgened (via module.IsCrossgened()), the NoSEH characteristic is added to indicate that the binary does not use Structured Exception Handling, which is important for proper PE header compliance.

This addresses issue #117752 by providing the actual implementation to ensure ILLink maintains proper PE header characteristics for crossgened modules.

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[agocke]

Fixes #117752

[agocke]

Please fix the compiler errors in the dotnet-linker-tests due to ambiguous namespaces

[Copilot]

Please fix the compiler errors in the dotnet-linker-tests due to ambiguous namespaces

Fixed the ambiguous namespace compiler errors by removing the broad using statements for System.Reflection.Metadata and System.Reflection.PortableExecutable that were conflicting with the existing Individual test namespaces. The PE types are now fully qualified in the test method to avoid ambiguity. (976e823)

[AaronRobinsonMSFT]

@copilot in OutputStep.cs, there is a function called WriteAssembly(). Please update it so that when the module is detected to be crossgened, it does the following module.Characteristics |= ModuleCharacteristics.NoSEH.

[Copilot]

@copilot in OutputStep.cs, there is a function called WriteAssembly(). Please update it so that when the module is detected to be crossgened, it does the following `module.Characteristics |= ModuleC...

Updated the WriteAssembly() method in OutputStep.cs to set module.Characteristics |= ModuleCharacteristics.NoSEH when a module is detected to be crossgened. The change is added alongside the existing attributes modification in the crossgened detection block. (8d3a5d8)

[Copilot]

Pull Request Overview

This PR implements a fix to ensure the NoSEH (No Structured Exception Handling) flag is properly set in PE headers when ILLink processes crossgened assemblies. The change addresses issue #117752 by maintaining proper PE header compliance for these modules.

Key changes:

• Added NoSEH characteristic flag setting for crossgened modules in ILLink's output step
• Ensures PE header compliance alongside existing IL-only attributes

[agocke]

Thanks for pushing this along. I was trying to add a test case, but it was a pain to get R2R to run during ILLink testing.