Restore E_NOTIMPL fallback in DacDbiInterfaceImpl::GetContext (#128499)

On platforms where ICorDebugDataTarget::GetThreadContext returns
E_NOTIMPL, restore the fallback that walks the thread's Frame chain to
reconstruct a register context from REGDISPLAY. This is needed for
managed debugging stack walks on non-Windows platforms.

The fallback was removed in 5391e9e.

Found using internal Visual Studio testing

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: tommcdon

src/coreclr/debug/daccess/dacdbiimpl.cpp

@@ -5682,10 +5682,57 @@ HRESULT STDMETHODCALLTYPE DacDbiInterfaceImpl::GetContext(VMPTR_Thread vmThread,
         {
             // If the filter context is NULL, then we use the true context of the thread.
             pContextBuffer->ContextFlags = DT_CONTEXT_ALL;
-            IfFailThrow(m_pTarget->GetThreadContext(pThread->GetOSThreadId(),
+            HRESULT hrContext = m_pTarget->GetThreadContext(pThread->GetOSThreadId(),
                                                     pContextBuffer->ContextFlags,
                                                     sizeof(DT_CONTEXT),
-                                                    reinterpret_cast<BYTE *>(pContextBuffer)));
+                                                    reinterpret_cast<BYTE *>(pContextBuffer));
+            if (hrContext == E_NOTIMPL)
+            {
+                // GetThreadContext is not implemented on this data target (e.g. Linux).
+                // That's why we have to make do with context we can obtain from Frames explicitly stored in Thread object.
+                // It suffices for managed debugging stackwalk.
+                REGDISPLAY tmpRd = {};
+                T_CONTEXT tmpContext = {};
+                FillRegDisplay(&tmpRd, &tmpContext);
+
+                // Going through thread Frames and looking for first (deepest one) one that
+                // that has context available for stackwalking (SP and PC)
+                // For example: RedirectedThreadFrame, InlinedCallFrame, DynamicHelperFrame
+                Frame *frame = pThread->GetFrame();
+
+                while (frame != NULL && frame != FRAME_TOP)
+                {
+#ifdef FEATURE_INTERPRETER
+                    if (frame->GetFrameIdentifier() == FrameIdentifier::InterpreterFrame)
+                    {
+                        PTR_InterpreterFrame pInterpreterFrame = dac_cast<PTR_InterpreterFrame>(frame);
+                        pInterpreterFrame->SetContextToInterpMethodContextFrame(&tmpContext);
+                        CopyMemory(pContextBuffer, &tmpContext, sizeof(*pContextBuffer));
+                        return S_OK;
+                    }
+#endif // FEATURE_INTERPRETER
+                    frame->UpdateRegDisplay(&tmpRd);
+                    if (GetRegdisplaySP(&tmpRd) != 0 && GetControlPC(&tmpRd) != 0)
+                    {
+                        UpdateContextFromRegDisp(&tmpRd, &tmpContext);
+                        CopyMemory(pContextBuffer, &tmpContext, sizeof(*pContextBuffer));
+                        pContextBuffer->ContextFlags = DT_CONTEXT_CONTROL
+    #if defined(TARGET_AMD64) || defined(TARGET_ARM)
+                                                    | DT_CONTEXT_INTEGER
+    #endif
+                        ;
+                        return S_OK;
+                    }
+                    frame = frame->Next();
+                }
+
+                // It looks like this thread is not running managed code.
+                ZeroMemory(pContextBuffer, sizeof(*pContextBuffer));
+            }
+            else
+            {
+                IfFailThrow(hrContext);
+            }
         }
         else
         {