C++-ify parser_state

C++-ify parser_state

This mildly C++-ifies parser_state and stap_parse_info -- just enough
to remove some cleanups.

This version includes the changes implemented by Simon.

Regression tested by the buildbot.

gdb/ChangeLog
2017-12-30  Tom Tromey  <tom@tromey.com>
	    Simon Marchi  <simon.marchi@ericsson.com>

	* stap-probe.h (struct stap_parse_info): Add constructor,
	destructor.
	* stap-probe.c (stap_parse_argument): Update.
	* rust-exp.y (rust_lex_tests): Update.
	* parser-defs.h (struct parser_state): Add constructor,
	destructor, release method.
	<expout>: Change type to expression_up.
	(null_post_parser): Change type.
	(initialize_expout, reallocate_expout): Remove.
	* parse.c (parser_state::parser_state): Rename from
	initialize_expout.
	(parser_state::release): Rename from reallocate_expout.
	(write_exp_elt, parse_exp_in_context_1, increase_expout_size):
	Update.
	(null_post_parser): Change type of "exp".
	* dtrace-probe.c (dtrace_probe::build_arg_exprs): Update.
	* ada-lang.c (resolve, resolve_subexp)
	(replace_operator_with_call): Change type of "expp".
	* language.h (struct language_defn) <la_post_parser>: Change type
	of "expp".