Return E_UNEXPECTED instead of S_FALSE from GetInterpreterCallDvarOffset

Caller uses IfFailRet which treats S_FALSE as success. With the IsInterpreted() gating
in place, neither S_FALSE return path is reachable on the happy path, so propagate them
as real errors.

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

Author: kotlarmilos

src/coreclr/debug/di/module.cpp

@@ -5139,7 +5139,7 @@ HRESULT CordbNativeCode::GetInterpreterCallDvarOffset(ULONG32 ILoffset, ULONG32
         memcpy(&interpOpcode, nativeBuffer, sizeof(int32_t));
         int interpSlots = GetInterpreterCallOpcodeLengthInSlots(interpOpcode);
         if (interpSlots <= 0)
-            return S_FALSE; // Not an interpreter call.
+            return E_UNEXPECTED;
 
         ULONG32 expectedPostCall = pMap->nativeStartOffset + interpSlots * (ULONG32)sizeof(int32_t);
         if (expectedPostCall != postCallNativeOffset)
@@ -5151,7 +5151,7 @@ HRESULT CordbNativeCode::GetInterpreterCallDvarOffset(ULONG32 ILoffset, ULONG32
         return S_OK;
     }
 
-    return S_FALSE;
+    return E_UNEXPECTED;
 }
 #endif // FEATURE_INTERPRETER