gccrs: borrowck: extract regions from types using VA

gccrs: borrowck: extract regions from types using VA

Biggybag on variance analysis to extract regions of fields from ADT
regions.

gcc/rust/ChangeLog:

	* typecheck/rust-tyty-variance-analysis-private.h (class FieldVisitorCtx):
	Region extraction.
	* typecheck/rust-tyty-variance-analysis.cc (query_field_regions): Region extraction.
	(FieldVisitorCtx::collect_regions): Region extraction.
	(FieldVisitorCtx::add_constraints_from_ty): Region extraction.
	(FieldVisitorCtx::add_constraints_from_region): Region
	extraction.
	(FieldVisitorCtx::add_constrints_from_param): Region extraction.
	* typecheck/rust-tyty-variance-analysis.h (query_field_regions):
	Region extraction.

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