Remove some uses of VEC from parsers

Remove some uses of VEC from parsers

This changes some uses of VEC in a few parsers to std::vector instead.

Tested by the buildbot.

gdb/ChangeLog
2018-08-28  Tom Tromey  <tom@tromey.com>

	* c-exp.y (struct token_and_value): Remove typedef and DEF_VEC.
	(token_fifo): Now a std::vector.
	(yylex, c_parse): Update.
	* d-exp.y (struct token_and_value): Remove typedef and DEF_VEC.
	(token_fifo): Now a std::vector.
	(yylex, d_parse): Update.
	* go-exp.y (struct token_and_value): Remove typedef and DEF_VEC.
	(token_fifo): Now a std::vector.
	(yylex, go_parse): Update.