Convert ada-exp.y to use operations

Convert ada-exp.y to use operations

This converts the Ada parser to generate operations rather than
exp_elements.

This was the most difficult of the parser conversions, partly due to
the decision to integrate Ada expression resolution into the parse,
and partly due to Ada aggregregate assignment.  A couple of new
per-parse globals are introduced, along with a number of helper
functions.  Resolution is done in 'ada_pop', yielding the unfortunate
rule that ada-exp.y should generally not use parser_state::pop
(exceptions are marked).

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* ada-exp.y: Create operations.
	(empty_stoken): Remove.
	(ada_pop, ada_wrap, ada_addrof, ada_un_wrap2, ada_wrap2)
	(ada_wrap_op, ada_wrap3, ada_funcall): New functions.
	(components): New global.
	(push_component, choice_component, pop_component, pop_components):
	New functions.
	(associations): New global
	(push_association, pop_association, pop_associations): New
	functions.
	(ada_parse): Update.
	(write_var_from_sym, write_int): Create operations.
	(write_exp_op_with_string): Remove.
	(write_object_renaming, write_selectors, write_ambiguous_var)
	(write_var_or_type, write_name_assoc): Create operations.
	* ada-lang.h (ada_index_type): Declare.
	* ada-lang.c (ada_index_type): No longer static.