gccrs: borrowck: Fact collector

gccrs: borrowck: Fact collector

This is the main Polonius based logic which creates the information
Polonius needs from BIR. It is largly guessed and rever engineered, so
some aspects are probably wrong.

gcc/rust/ChangeLog:

	* checks/errors/borrowck/rust-bir-fact-collector.h: New file.
	* checks/errors/borrowck/rust-borrow-checker.cc (BorrowChecker::go):
	Enable fact collection.

Signed-off-by: Jakub Dupak <dev@jakubdupak.com>