Implement support for copying C++ classes. (#6434)

When performing impl lookup for `Core.Copy` for a C++ class type, look
for a copy constructor. If we find one, synthesize an impl witness that
calls the constructor.

This adds initial support for impl lookup to delegate to the C++ interop
logic for queries involving C++ types. For now, we don't implement the
rules from #6166 that compare a synthesized type structure for the C++
impl against the best Carbon type structure, but the framework for
building that support is established here.

Currently there is no caching of the lookup here, and we build unique
`ImplWitnessTable`s for each lookup, which leads to each impl lookup
producing a distinct facet value. This results in some errors in generic
contexts; this will be addressed in follow-up changes. This PR aims only
to support the non-generic case.

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>
Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>

toolchain/check/cpp/import.cpp

@@ -40,6 +40,7 @@
 #include "toolchain/check/convert.h"
 #include "toolchain/check/cpp/access.h"
 #include "toolchain/check/cpp/custom_type_mapping.h"
+#include "toolchain/check/cpp/location.h"
 #include "toolchain/check/cpp/macros.h"
 #include "toolchain/check/cpp/thunk.h"
 #include "toolchain/check/diagnostic_helpers.h"
@@ -1809,6 +1810,11 @@ static auto ImportFunctionDecl(Context& context, SemIR::LocId loc_id,
         function_info.SetHasCppThunk(thunk_function_decl_id);
       }
     }
+  } else {
+    // Inform Clang that the function has been referenced. This will trigger
+    // instantiation if needed.
+    context.clang_sema().MarkFunctionReferenced(GetCppLocation(context, loc_id),
+                                                clang_decl);
   }
 
   return function_info.first_owning_decl_id;