gccrs: unify how we handle DST's

gccrs: unify how we handle DST's

DST's are not truely reference types they are "unsized types" so
the exact size of them is not known at compile time. We actually
achieve this by pretending they are a reference but really its
struct we pass around.

Fixes #2180

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::visit): refactr
	(CompileExpr::get_fn_addr_from_dyn): likewise
	(CompileExpr::get_receiver_from_dyn): likewise
	(CompileExpr::type_cast_expression): likewise
	* backend/rust-compile-type.cc (TyTyResolveCompile::visit): likewise
	(TyTyResolveCompile::create_dyn_obj_record): likewise
	(TyTyResolveCompile::create_slice_type_record): likewise
	(TyTyResolveCompile::create_str_type_record): likewise
	* backend/rust-compile-type.h: likewise
	* backend/rust-compile.cc (HIRCompileBase::coercion_site1): likewise
	(HIRCompileBase::coerce_to_dyn_object): refactor
	* backend/rust-tree.h (SLICE_FLAG): removed
	(SLICE_TYPE_P): removed
	(RS_DST_FLAG): new flag
	(RS_DST_FLAG_P): new predicate
	* typecheck/rust-tyty.cc (ReferenceType::is_dyn_object): new helper
	(ReferenceType::is_dyn_obj_type): likewise
	(PointerType::is_dyn_object): likewise
	(PointerType::is_dyn_obj_type): likewise
	* typecheck/rust-tyty.h (class DynamicObjectType): moved up

gcc/testsuite/ChangeLog:

	* rust/execute/torture/issue-2180.rs: New test.

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