Remove some explicit memory management from dwarf2read.c

Remove some explicit memory management from dwarf2read.c

I noticed a few spots in dwarf2read.c that could be improved by moving
to unique_xmalloc_ptr or, in one case, std::vector.

gdb/ChangeLog
2020-01-08  Tom Tromey  <tromey@adacore.com>

	* dwarf2read.c (add_partial_symbol): Use unique_xmalloc_ptr.
	(dwarf2_compute_name, open_dwo_file): Likewise.
	(process_enumeration_scope): Use std::vector.
	(guess_partial_die_structure_name): Use unique_xmalloc_ptr.
	(partial_die_info::fixup, dwarf2_start_subfile)
	(guess_full_die_structure_name, dwarf2_name): Likewise.
	(determine_prefix): Update.
	(guess_full_die_structure_name): Make return type const.
	(partial_die_full_name): Return unique_xmalloc_ptr.
	(DW_FIELD_ALLOC_CHUNK): Remove.

Change-Id: I1cb278c608041ef36ef1f77c7e7565c921038d08