Remove a cleanup from parse_expression_for_completion

This removes a cleanup from parse_expression_for_completion, by
changing various expression-completion functions to use
gdb::unique_xmalloc_ptry rather than explicit malloc+free.

Regression tested by the buildbot.

gdb/ChangeLog
2018-02-21  Tom Tromey  <tom@tromey.com>

	* value.h: (extract_field_op): Update.
	* eval.c (extract_field_op): Return a const char *.
	* expression.h (parse_expression_for_completion): Update.
	* completer.c (complete_expression): Update.
	(add_struct_fields): Make fieldname const.
	* parse.c (expout_completion_name): Now a unique_xmalloc_ptr.
	(mark_completion_tag, parse_exp_in_context_1): Update.
	(parse_expression_for_completion): Change "name" to
	unique_xmalloc_ptr*.