gccrs: expand: Move cfg-attrs related functions out of MacroExpander

gccrs: expand: Move cfg-attrs related functions out of MacroExpander

The current situation where the MacroExpander and AttrVisitor recursively
call into each other is not great, and causes pains for macro imports. We
need to split this pass in two, with one pass being responsible for cfg-attr
checking and stripping, and one being responsible for macro expansion.

gcc/rust/ChangeLog:

	* expand/rust-macro-expand.cc (MacroExpander::expand_crate): Do not cfg-attr
	strip in MacroExpander
	(MacroExpander::fails_cfg): Function moved...
	(MacroExpander::fails_cfg_with_expand): Function moved...
	(MacroExpander::expand_cfg_attrs): Function moved...
	* expand/rust-attribute-visitor.cc (fails_cfg): ...here.
	(fails_cfg_with_expand): ...here.
	(expand_cfg_attrs): ...here.
	(AttrVisitor::expand_struct_fields): Use new functions.
	(AttrVisitor::expand_tuple_fields): Likewise.
	(AttrVisitor::expand_function_params): Likewise.
	(AttrVisitor::visit): Likewise.
	(AttrVisitor::go): New function.
	* expand/rust-attribute-visitor.h: Declare AttrVisitor::go.
	* expand/rust-macro-expand.h (struct MacroExpander): Remove cfg-attr related
	functions.