Don't make an extra copy + allocation of the demangled name

Don't make an extra copy + allocation of the demangled name

We can just keep around the malloc()-ed name we got from bfd and free
it later.

gdb/ChangeLog:

2019-10-25  Christian Biesinger  <cbiesinger@google.com>

	* symtab.c (struct demangled_name_entry): Change demangled name
	to a unique_xmalloc_ptr<char>, now that we don't allocate it as
	part of the struct anymore.
	(symbol_set_names): No longer obstack allocate + copy the demangled
	name, just store the allocated name from bfd.

Change-Id: Ie6ad50e1e1e73509f55d756f0a437897bb93e3b0