Strong type FreeRegion

Strong type FreeRegion

gcc/rust/ChangeLog:

	* checks/errors/borrowck/rust-bir-builder-internal.h: Use
	STATIC_FREE_REGION, use value of FreeRegion for origin.
	* checks/errors/borrowck/rust-bir-builder.h: Use free region
	value.
	* checks/errors/borrowck/rust-bir-dump.cc (Dump::visit_scope):
	Likewise.
	* checks/errors/borrowck/rust-bir-fact-collector.h (points):
	Likewise.
	* checks/errors/borrowck/rust-bir-free-region.h (struct FreeRegion):
	Make FreeRegion a struct.
	* checks/errors/borrowck/rust-bir-place.h: Use FreeRegion
	instead of Origin in PlaceDB.
	* typecheck/rust-tyty-variance-analysis.cc (FieldVisitorCtx::add_constraints_from_region):
	Use value of FreeRegion for origin.
	(FieldVisitorCtx::add_constrints_from_param): Likewise.
	(Term::make_transform): Likewise.

Signed-off-by: Kushal Pal <kushalpal109@gmail.com>