gccrs: expand: Expand item level attribute proc macros

gccrs: expand: Expand item level attribute proc macros

Expand custom attributes from procedural macros at item level.

gcc/rust/ChangeLog:

	* expand/rust-expand-visitor.cc (expand_attribute): Add function
	to expand a given attribute on a given item.
	* expand/rust-macro-expand.h (struct MacroExpander): Change
	return type to ast fragment.

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