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/function.cpp

@@ -140,9 +140,9 @@ static auto MakeFunctionSignature(Context& context, SemIR::LocId loc_id,
         context, context.types().GetTypeInstId(args.self_type_id));
     EndEmptySubpattern(context);
 
-    insts.self_param_id = AddParamPattern(
-        context, loc_id, SemIR::NameId::SelfValue, self_type_region_id,
-        args.self_type_id, args.self_is_ref);
+    insts.self_param_id =
+        AddParamPattern(context, loc_id, SemIR::NameId::SelfValue,
+                        self_type_region_id, args.self_type_id, args.self_kind);
     insts.implicit_param_patterns_id =
         context.inst_blocks().Add({insts.self_param_id});
 
@@ -164,7 +164,7 @@ static auto MakeFunctionSignature(Context& context, SemIR::LocId loc_id,
 
       context.inst_block_stack().AddInstId(AddParamPattern(
           context, loc_id, SemIR::NameId::Underscore, param_type_region_id,
-          param_type_id, /*is_ref=*/args.params_are_refs));
+          param_type_id, args.param_kind));
     }
     insts.param_patterns_id = context.inst_block_stack().Pop();
   }