Support const eval when calling a C++ thunk (#6947)

This makes it possible to do const eval when calling a constexpr C++
function with params and return types other than 32/64-bit integers.

Most of the new logic is in `MaybeModifyCppThunkCallForConstEval`, which
is called by `MakeConstantForCall`. This checks if the callee is a C++
thunk (using a new `SpecialFunctionKind::CppThunk` variant), and if so
it:
* Changes the callee from the C++ thunk to the thunk's callee
* Remaps parameters that are passed by pointer to the thunk to the
underlying value
* Drops the return value parameter, if present

toolchain/check/cpp/import.cpp

@@ -1587,8 +1587,10 @@ static auto ImportFunctionDecl(Context& context, SemIR::LocId loc_id,
               context, loc_id, import_ir_inst_id, thunk_clang_decl,
               {.num_params =
                    static_cast<int32_t>(thunk_clang_decl->getNumParams())})) {
+        auto& thunk_function = context.functions().Get(*thunk_function_id);
+        thunk_function.SetCppThunk(function_info.first_owning_decl_id);
         SemIR::InstId thunk_function_decl_id =
-            context.functions().Get(*thunk_function_id).first_owning_decl_id;
+            thunk_function.first_owning_decl_id;
         function_info.SetHasCppThunk(thunk_function_decl_id);
       }
     }