Minor Ada-related cleanups

This patch addresses some review comments that I forgot to deal with
in an earlier patch.  See the comments here:

https://sourceware.org/pipermail/gdb-patches/2021-February/176278.html

For the most part this is fixing up comments, but it also includes
adding a constructor and initializers to "match_data".

Regression tested on x86-64 Fedora 32.

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

	* ada-lang.c (ada_resolve_function): Update comment.
	(is_nonfunction, add_symbols_from_enclosing_procs)
	(remove_extra_symbols): Likewise.
	(struct match_data): Add constructor, initializers.
	(add_nonlocal_symbols): Remove memset.
	(aux_add_nonlocal_symbols): Update comment.
	(ada_add_block_renamings, add_nonlocal_symbols)
	(ada_add_all_symbols): Likewise.
	* ada-exp.y (write_var_or_type): Clean up trailing whitespace.