[gdb] Fix regression in dwarf2_name

[gdb] Fix regression in dwarf2_name

Since commit 2c830f5475 "Change some uses of DW_STRING to string method" we
have these regressions:
...
FAIL: gdb.base/info-types-c++.exp: info types
FAIL: gdb.cp/anon-struct.exp: print type of t::t
FAIL: gdb.cp/anon-struct.exp: print type of X::t2
FAIL: gdb.cp/anon-struct.exp: print type of X::t2::t2
FAIL: gdb.cp/anon-struct.exp: print type of t3::~t3
...

Fix these in dwarf2_name by updating attr_name each time attr is updated.

Tested on x86_64-linux.

gdb/ChangeLog:

2020-09-30  Tom de Vries  <tdevries@suse.de>

	PR symtab/26683
	* dwarf2/read.c (dwarf2_name): Update attr_name after attr is updated.