gccrs: Handle attributes in expression macros

gccrs: Handle attributes in expression macros

gcc/rust/ChangeLog:

	* expand/rust-macro-expand.cc (transcribe_expression): Parse any
	outer attributes before parsing an expression.
	* parse/rust-parse.h (Parser::parse_outer_attributes): Make
	public.

gcc/testsuite/ChangeLog:

	* rust/compile/attr-macro.rs: New test.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>