C++-ify Ada component interval handling

C++-ify Ada component interval handling

The Ada component interval handling code, used for aggregate
assignments, does a pre-pass over the sub-expressions so that it can
size an array.  For my expression rewrite, it was handy to C++-ify
this.

gdb/ChangeLog
2020-12-15  Tom Tromey  <tom@tromey.com>

	* ada-lang.c (num_component_specs): Remove.
	(assign_aggregate): Update.
	(aggregate_assign_positional, aggregate_assign_from_choices)
	(aggregate_assign_others, add_component_interval): Change
	arguments.