gccrs: Parse unparsed derive tokenstreams

gccrs: Parse unparsed derive tokenstreams

Derive attributes should be parsed before attempting to retrieve any
traits. This will convert the tokenstream to a list of path if this
hasn't been done previously.

gcc/rust/ChangeLog:

	* ast/rust-ast.cc (Attribute::get_traits_to_derive): Convert
	tokenstream to path list.

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