[clr-interp] Fix some interop suite issues

[davidwrighton]

This fixes several issues that were blocking the interop suite from passing:

• Platform detection of IsBuiltInComEnabled in the presence of interpreter support
• Move delegate creation to use the optimized constructors. This works around creating a delegate to a function which is marked with [UnmanagedCallersOnly]. We've since decided to fix the test suite, but this is an improvement to the compiler in any case.

[davidwrighton]

@AaronRobinsonMSFT @jkoritzinsky do you have an opinion on the check of HasUnmanagedCallersOnlyAttribute in the delegate constructor. We currently have a check for it in optimized delegate construction, which is almost always used by the JIT, but I need to know if we really need that to work, or if it's just a convenience feature that it fails.

[davidwrighton]

/azp run runtime-interpreter

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[davidwrighton]

/azp run runtime-interpreter

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Azure Pipelines successfully started running 1 pipeline(s).

[dotnet-policy-service]

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[Copilot]

Pull request overview

This PR fixes several interop suite issues related to the CoreCLR interpreter. The main changes include: (1) correcting platform detection for built-in COM support when the interpreter is active, and (2) implementing a peephole optimization for delegate constructor patterns to use optimized constructors.

Key changes:

• Modified IsBuiltInComEnabled to return false when the CoreCLR interpreter is active
• Added a peephole optimizer that recognizes LDFTN/LDVIRTFTN + NEWOBJ patterns and rewrites them to use optimized delegate constructors
• Refactored ldftn-related code into reusable helper methods

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
InvalidSafeHandleMarshallingTests.cs	Adds test skip attribute for interpreter mode with comment explaining COM interop behavior
PlatformDetection.cs	Modifies IsBuiltInComEnabled to exclude CoreCLR interpreter from COM support detection
compiler.h	Adds member variable m_pConstrainedToken and declares new helper methods for ldftn operations and delegate constructor optimization
compiler.cpp	Implements delegate constructor peephole optimization, refactors ldftn/ldvirtftn code into EmitLdftn, extracts EmitDup and EmitLoadPointer helpers, and converts constrained token handling from local to member variable

[janvorli]

LGTM, thank you!

[davidwrighton]

/ba-g known issues + a machine that would have produced a known issue instead dead-lettered.