Import non-const rvalue references as var parameters. (#7125)

When importing a C++ function with an rvalue reference parameter, we
previously produced a Carbon value parameter. This would lead to the
toolchain believing it could pass the address of a non-expiring object
to the function, which would lead to a use-after-move.

Instead, we now map non-const rvalue reference parameters to Carbon
`var` parameters. This forces the object passed into C++ to be unique
and owned by the call. While that's not an exact match for C++ rvalue
reference parameters, given that it provides "always move" not
"conditionally move", it's the closest match we have at the moment.

toolchain/check/cpp/operators.cpp

@@ -15,6 +15,7 @@
 #include "toolchain/check/cpp/type_mapping.h"
 #include "toolchain/check/function.h"
 #include "toolchain/check/inst.h"
+#include "toolchain/check/pattern.h"
 #include "toolchain/check/type.h"
 #include "toolchain/check/type_completion.h"
 #include "toolchain/sem_ir/builtin_function_kind.h"
@@ -266,6 +267,7 @@ static auto MakeCppStdInitializerListMake(Context& context, SemIR::LocId loc_id,
                                 {.parent_scope_id = init_list_class.scope_id,
                                  .name_id = init_list_class.name_id,
                                  .param_type_ids = {array_type_id},
+                                 .param_kind = ParamPatternKind::Value,
                                  .return_type_id = init_list_type_id});
 
   auto& function = context.functions().Get(function_id);