Changed DW_AT_string_length semantic to reflect Dwarf
 standard.

gdb/:

2013-07-23  Keven Boell  <keven.boell@intel.com>

	* dwarf2read.c (read_tag_string_type): changed
	semantic of DW_AT_string_length to be able to
	handle Dwarf blocks as well. Support for
	DW_AT_byte_length added to get correct length
	if specified in combination with
	DW_AT_string_length.
	* dwarf2read.c (attr_to_locexprbaton): added
	functionality to add Dwarf operators to baton
	data attribute.

Change-Id: I205df4f52e3ff44000cd751d01468d80da4bf1ec
Signed-off-by: Keven Boell <keven.boell@intel.com>