Allow multiple outer attributes on generic params

Allow multiple outer attributes on generic params

Previously generic params only allowed one outer attribute in front of
them.

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc (TokenCollector::visit): Visit outer
	attributes.
	* ast/rust-ast-visitor.cc (DefaultASTVisitor::visit): Change outer
	attribute visit, we need to visit all of them.
	* ast/rust-ast.cc (LifetimeParam::as_string): Change as_string
	implementation to allow multiple outer attributes.
	(TypeParam::as_string): Likewise.
	* ast/rust-ast.h (class LifetimeParam): Allow multiple outer
	attributes.
	* ast/rust-item.h (class TypeParam): Likewise.
	* ast/rust-path.h: Likewise.
	* parse/rust-parse-impl.h (Parser::parse_generic_param): Change call
	to outer attribute parsing to collect several attributes.
	(Parser::parse_lifetime_param): Likewise.
	(Parser::parse_type_param): Likewise.

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