Add a section about shadowed variables and host compiler approach

Author: AlexeySachkov

sycl/doc/design/MappingHostAddressesToDeviceEntities.md

@@ -319,3 +319,26 @@ Generated constructor contains a call to
 `__register_uniquely_identifiable_object` for each global variable which has
 `sycl-unique-id` and `sycl-uid-kind` attributes, passing values of those
 attributes into the corresponding arguments of the function.
+
+### Handling shadowed variables
+
+Unlike with the integration footer the problem with shadowed variables doesn't
+really exists with the custom host compiler approach, because it is compiler
+responsibility to uniquely identify shadowed variables at LLVM IR level and we
+are simply re-using what is already there.
+
+For example, for the following code snippet:
+
+```
+sycl::device_global<int> FuBar;
+namespace {
+  sycl::device_global<int> FuBar;
+}
+```
+
+The following IR is generated by our host compiler:
+
+```
+@FuBar = dso_local global %"class.cl::sycl::ext::oneapi::device_global" zeroinitializer, align 8
+@_ZN12_GLOBAL__N_15FuBarE = internal global %"class.cl::sycl::ext::oneapi::device_global" zeroinitializer, align 8
+```