gdb/

gdb/
	Remove excessive DWARF expressions memory duplication.
	* dwarf2loc.c (per_cu_dwarf_call): Remove variable back_to and its use
	for block.data.
	(indirect_pieced_value): Remove variable result.  Remove variable
	back_to and its use for baton.data.
	(dwarf2_compile_expr_to_ax): Remove variable back_to and its use for
	block.data.
	* dwarf2read.c (dwarf2_fetch_die_location_block): Remove xmemdup.
	Update the function comment.