[clr-interp] Implement Managed Return Value support for the interpreter

[kotlarmilos]

Description

This PR adds managed return value support for interpreted methods so the debugger can surface the value a method just returned when stepping over or out of a call. Building on the variable-home contract from #128479 where EmitCall flags each value-returning IL call/callvirt site. The interpreter stores every return value in that slot, so the generic DI consumer reads it like any other variable home with no interpreter-specific disassembly or ABI logic.

Testing

Ran the MRV debugger tests against a local Checked CoreCLR build with DOTNET_InterpMode=3 and all active MRV tests pass under the interpreter:

• MRV.TestArrays
• MRV.TestAsyncMethods
• MRV.TestClasses
• MRV.TestComplexGenerics
• MRV.TestEnums
• MRV.TestGenerics
• MRV.TestMisc
• MRV.TestNativeCalls
• MRV.TestPointers
• MRV.TestPrimitiveTypes
• MRV.TestReflectionWithAbstractMethod
• MRV.TestRefs
• MRV.TestStructs

[dotnet-policy-service]

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

[Copilot]

Pull request overview

This PR extends CoreCLR interpreter debug info so managed return values can be surfaced for interpreted calls, and broadens IL/native mapping beyond stack-empty offsets. It fits into the debugger/interpreter pipeline by teaching the interpreter to emit richer boundary data and teaching DI to decode interpreter callsites.

Changes:

• Adds interpreter-side IL/native map capacity tracking and emits per-IL-offset mappings plus CALL_INSTRUCTION entries for interpreter call opcodes.
• Adds DI-side interpreter opcode-length decoding and synthetic stack-home lookup for interpreter return values.
• Wires CordbJITILFrame to read interpreted return values from FP-relative dvars instead of native return registers.

Review found a blocking correctness issue: the new CALL_INSTRUCTION tagging in compiler.cpp also marks compiler-inserted helper calls (for example the helper emitted by EmitPushLdvirtftn), so GetReturnValueLiveOffset can report extra offsets for a single IL callsite and one of those offsets corresponds to the helper’s function-pointer result rather than the user call’s return value.

Show a summary per file

File	Description
src/coreclr/interpreter/compiler.h	Adds native-map capacity tracking for interpreter-emitted debug boundaries.
src/coreclr/interpreter/compiler.cpp	Emits expanded IL/native mappings and interpreter callsite markers.
src/coreclr/debug/di/rsthread.cpp	Reads interpreted return values from synthetic FP-relative variable homes.
src/coreclr/debug/di/rspriv.h	Declares the new interpreter-specific DI helper.
src/coreclr/debug/di/module.cpp	Decodes interpreter callsite lengths and dvar offsets in DI.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 3

[Copilot]

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 1

[noahfalk]

@kotlarmilos - If possible lets hold off on this one until we settle what data contract we want here overall. If getting this in is blocking making progress elsewhere let me know but I'm guessing its relatively isolated. If we do wind up embedding this interpreter info into the debugger code we'll probably want to define some new IDacDbiInterface APIs and put the logic that understands interpreter disassembly in the DAC instead.

[Copilot]

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1

[kotlarmilos]

@kotlarmilos - If possible lets hold off on this one until we settle what data contract we want here overall. If getting this in is blocking making progress elsewhere let me know but I'm guessing its relatively isolated. If we do wind up embedding this interpreter info into the debugger code we'll probably want to define some new IDacDbiInterface APIs and put the logic that understands interpreter disassembly in the DAC instead.

This PR is rebased on #128479, in which EmitCall flags each IL call/callvirt site that returns a value.

[Copilot]

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0 new

[noahfalk]

Looks great!