Here we wrongly mark the reference temporary for g TREE_READONLY,
so it's put in .rodata and so we can't modify its subobject even
when the subobject is marked mutable. This is so since r9-869.
r14-1785 fixed a similar problem, but not in set_up_extended_ref_temp.
PR c++/116369
gcc/cp/ChangeLog:
* call.cc (set_up_extended_ref_temp): Don't mark a temporary
TREE_READONLY if its type is TYPE_HAS_MUTABLE_P.