gccrs: Handle gengtype annotations in backend/rust-tree.{cc,h}

gccrs: Handle gengtype annotations in backend/rust-tree.{cc,h}

gcc/rust/ChangeLog:

	* config-lang.in:
	Add "backend/rust-tree.h" and "backend/rust-tree.h" to gtfiles.
	* backend/rust-tree.cc: Include new header generated by gengtype.

	* backend/rust-tree.h
	(struct language_function): Add TODO.
	* rust-lang.cc:
	Include "rust-tree.h".
	(struct lang_type): Remove duplicate definition.
	(struct lang_decl): Likewise.
	(struct lang_identifier): Likewise.
	(struct language_function): Likewise.

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