Remove error state for GenericArg

Remove error state for GenericArg

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc (TokenCollector::visit): Remove error kind
	and change function call.
	* ast/rust-ast-visitor.cc (DefaultASTVisitor::visit): Change call name.
	* ast/rust-path.cc (ConstGenericParam::as_string): Likewise.
	* ast/rust-path.h: Remove error kind.
	* hir/rust-ast-lower-type.cc (ASTLowerGenericParam::visit): Change call
	name.
	* parse/rust-parse-impl.h (Parser::parse_generic_param): Use optional
	on parsing failure.
	(Parser::parse_generic_arg): Likewise.
	(Parser::parse_path_generic_args): Likewise.
	* parse/rust-parse.h: Likewise.
	* resolve/rust-ast-resolve-type.h: Change call name.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>