gccrs: Improve handling of implicit Self parameter in AST

gcc/rust/ChangeLog:

	* ast/rust-item.h
	(Trait::self_param): Add.
	(Trait::Trait): Initialize self_param.
	(Trait::operator=): Copy self_param.
	(Trait::insert_implicit_self): Remove.
	(Trait::get_implicit_self): Add.
	* hir/rust-ast-lower-item.cc
	(ASTLoweringItem::visit): Make sure implicit self is still
	lowered to HIR.
	* resolve/rust-ast-resolve-item.cc
	(ResolveItem::visit): Adjust handling of implicit self.
	* resolve/rust-early-name-resolver.cc
	(EarlyNameResolver::visit): Add commit to Trait visitor
	mentioning that implicit self is not visited.
	* resolve/rust-toplevel-name-resolver-2.0.cc
	(TopLevel::visit): Remove call to Trait::insert_implicit_self.

gcc/testsuite/ChangeLog:

	* rust/compile/nr2/exclude: Remove entries.
	* rust/link/generic_function_0.rs: No longer expect failure.
	* rust/link/trait_import_0.rs: Likewise.
	* rust/link/trait_import_1.rs
	(trait Sized): Add.

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