[clr-interp] Add various tweaks to allow code patterns around byref usage that are tested by various IL tests

[davidwrighton]

Allow use of the transient pointer concept from ECMA 335 using a "bashing" approach where the StackType is adjusted to I as needed. This is done at the same points that the JIT does this bashing. See I.12.3.2.1

Allow the convert opcode to operate on StackTypeByRef and StackTypeO as if they were of StackTypeI.

In combination with @janvorli's PR to implement JMP this should fix much of Directed_3 and IL_Conformance

[Copilot]

Pull Request Overview

This pull request implements support for transient pointers as defined in ECMA-335 section I.12.3.2.1 within the CoreCLR interpreter. The changes introduce a new stack type StackTypeTransientPointer to track when a byref pointer is "transient" (e.g., from ldloca operations), allowing more accurate type tracking and proper handling during operations like arithmetic, conversions, and comparisons.

Key changes:

• Added StackTypeTransientPointer enum value and infrastructure for tracking transient pointer state
• Encapsulated direct access to the type field in StackInfo behind a GetStackType() accessor
• Added methods to manage transient pointer state (SetAsTransientPointer, IsTransientPointer, BashStackTypeToI_ForTransientPointerUse, BashStackTypeToIForConvert)

Reviewed Changes

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

File	Description
src/coreclr/interpreter/compiler.h	Adds StackTypeTransientPointer enum value, makes StackInfo::type private with accessor methods, and adds transient pointer management methods
src/coreclr/interpreter/compiler.cpp	Updates all direct accesses to .type field to use GetStackType(), adds transient pointer handling logic in arithmetic/conversion operations, marks ldloca results as transient pointers

Comments suppressed due to low confidence (1)

src/coreclr/interpreter/compiler.cpp:1818

• The EmitConv function creates a new StackInfo with placement new, which always sets the internal type field directly via the constructor, bypassing transient pointer tracking. If a transient pointer undergoes conversion, its transient state will be lost after EmitConv. This could cause incorrect behavior when a transient pointer is converted and then needs to be treated as a regular byref or integer. Consider preserving transient pointer state where appropriate, or ensuring that BashStackTypeToIForConvert is called before conversions that should clear this state.

    new (sp) StackInfo(type, NULL, var);