Introduce dw2_linkage_name and dw2_linkage_name_attr.

Introduce dw2_linkage_name and dw2_linkage_name_attr.

The DWARF reader is littered with the following idiom to read a linkage name
from the debug info:

  mangled = dwarf2_string_attr (die, DW_AT_linkage_name, cu);
  if (mangled == NULL)
    mangled = dwarf2_string_attr (die, DW_AT_MIPS_linkage_name, cu);

This patch introduces functions to simplify this to:

  mangled = dw2_linkage_name (die, cu);

or

  attr = dw2_linkage_name_attr (die, cu);

gdb/ChangeLog:

	* dwarf2read.c (dw2_linkage_name_attr): New function.
	(dw2_linkage_name): New function.
	(dwarf2_compute_name, dwarf2_physname, read_call_site_scope)
	(guess_full_die_structure_name, dwarf2_name): Use dw2_linkage_name.
	(anonymous_struct_prefix, dwarf2_name): Use dw2_linkage_name_attr.