gccrs: Give the builtin unit struct an actual locus

gccrs: Give the builtin unit struct an actual locus

This has been a pet peeve of mine for a while because the gimple never
emitted the struct () name properly it was always empty which for record
types they always require a real locus or they dont get a proper name.

gcc/rust/ChangeLog:

	* backend/rust-compile-base.cc (HIRCompileBase::unit_expression): pass ctx
	* backend/rust-compile-base.h: cant be static
	* backend/rust-compile-intrinsic.cc (try_handler_inner): pass ctx
	* backend/rust-compile-type.cc
	(TyTyResolveCompile::get_unit_type): update to grab the first locus
	(TyTyResolveCompile::visit): pass ctx
	* backend/rust-compile-type.h: likewise

Signed-off-by: Philip Herron <herron.philip@googlemail.com>