Remove string-related cleanup from c-exp.y

Remove string-related cleanup from c-exp.y

This removes a string-related cleanup from c-exp.y, by adding a new
member to c_parse_state to store the strings.

gdb/ChangeLog
2019-01-06  Tom Tromey  <tom@tromey.com>

	* c-exp.y (struct c_parse_state) <strings>: New member.
	(operator_stoken): Update.