Add outer attributes to struct expr fields

Add outer attributes to struct expr fields

Struct fields can have outer attributes on their field for various
purpose, this behavior should be reflected upon struct expr fields.

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc (TokenCollector::visit): Output field
	attributes.
	* ast/rust-expr.h (class StructExprField): Add outer attributes member.
	* parse/rust-parse-impl.h (Parser::parse_struct_expr_field): Parse
	outer attributes and store them in the appropriate AST node.

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