[interp] Implement pinned IL locals

[kg]

Fixes JIT\Directed\pinning\object-pin\object-pin

[Copilot]

Pull Request Overview

This PR implements support for pinned IL locals in the CoreCLR interpreter by adding a pinned flag to track local variables that have the 'pinned' modifier and ensuring they are properly marked in the GC info.

• Adds a pinned bit field to the InterpVar structure to track pinned variables
• Updates local variable creation to detect and store the pinned modifier from IL
• Modifies GC info generation to mark pinned variables with the GC_SLOT_PINNED flag

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/coreclr/interpreter/compiler.h	Adds pinned field to InterpVar struct and updates CreateNextLocalVar signature
src/coreclr/interpreter/compiler.cpp	Implements pinned variable detection, storage, and GC info marking

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