c++: mutable temps in rodata

If the type of a temporary has mutable members, we can't set TREE_READONLY
on the VAR_DECL; this is parallel to the check in
cp_apply_type_quals_to_decl.

gcc/cp/ChangeLog:

	* tree.cc (build_target_expr): Check TYPE_HAS_MUTABLE_P.

gcc/testsuite/ChangeLog:

	* g++.dg/tree-ssa/initlist-opt6.C: New test.