[clr-interp] allow modification of the this pointer in functions which use it as the generics context

[davidwrighton]

• When the this pointer is used as the generics context, if some code modifies the this pointer, do not allow that modification to affect the generics behavior of the function
• Implement this by making a shadow copy of the this pointer in this situation
• To avoid doing this ALL of the time, I've made a scheme where we reserve space for the shadow copy var during normal var processing, but we fill it only if needed, and otherwise bash the shadow copy var to point to its natural location.
• Also handle the case of a legitimate null this pointer when doing EH type resolution, and have it throw a NullReferenceException.

This is a new PR replacing #119338, since I fouled up a merge in that PR.

[Copilot]

Pull Request Overview

This PR implements a fix for CLR interpreter's handling of generic methods that use the "this" pointer as the generics context. The change addresses a scenario where modifications to the "this" pointer during method execution could incorrectly affect the generics behavior.

Key Changes:

• Creates a shadow copy of the "this" pointer when it's used as the generics context in generic methods
• Implements lazy evaluation - reserves space for the shadow copy but only uses it if the "this" pointer is actually modified
• Adds null reference checking for the generics context when derived from "this"

Reviewed Changes

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

File	Description
src/coreclr/vm/codeman.cpp	Adds null reference checking for "this"-derived generics context
src/coreclr/interpreter/compiler.h	Adds tracking variables for shadow copy state
src/coreclr/interpreter/compiler.cpp	Implements shadow copy logic, variable management, and GC reporting

[dotnet-policy-service]

Tagging subscribers to this area: @BrzVlad, @janvorli, @kg
See info in area-owners.md if you want to be subscribed.

[janvorli]

LGTM, thank you!