Improve canonical path handling for impl items

Improve canonical path handling for impl items

gcc/rust/ChangeLog:

	* resolve/rust-ast-resolve-item.cc
	(ResolveItem::visit): Use the return values of
	CanonicalPath::inherent_impl_seg and
	CanonicalPath::trait_impl_projection_seg more directly.
	* util/rust-canonical-path.h
	(CanonicalPath::trait_impl_projection_seg): Append "<impl "
	instead of "<" to the beginning of the returned path segment.
	(CanonicalPath::inherent_impl_seg): Likewise.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>