Fix some comment nits

Fix some comment nits

While studying the TYPE_CANONICAL/TYPE_STRUCTURAL_EQUALITY_P stuff,
I've noticed some nits in comments, the following patch fixes them.

2024-04-16  Jakub Jelinek  <jakub@redhat.com>

	* tree.cc (array_type_nelts): Ensure 2 spaces after . in comment
	instead of just one.
	(build_variant_type_copy): Likewise.
	(tree_check_failed): Likewise.
	(build_atomic_base): Likewise.
	* ipa-free-lang-data.cc (fld_incomplete_type_of): Use an indefinite
	article rather than a.