gccrs: Add outer attributes to struct expr fields

gccrs: 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>