gccrs: privacy: Refactor proc macro privacy check

gccrs: privacy: Refactor proc macro privacy check

Refactor proc macro specific privacy check in multiple shorter
functions.

gcc/rust/ChangeLog:

	* checks/errors/privacy/rust-privacy-reporter.cc (find_proc_macro_attribute):
	Add a new function to find a potential proc macro type
	attribute on a given item.
	(proc_macro_privacy_check): Move all proc macro privacy check in
	their own function to avoid cluttering the usual privacy check.
	(PrivacyReporter::go): Add call to newly created proc macro
	privacy check function.

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