gccrs: Change lookup_hir_to_node return type to optional

gccrs: Change lookup_hir_to_node return type to optional

Optional are more convenient to use and avoid uninitialized data.

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::generate_closure_function):
	Adapt code for new optional return type.
	* checks/errors/privacy/rust-privacy-reporter.cc (PrivacyReporter::check_base_type_privacy):
	Likewise.
	* util/rust-hir-map.cc (Mappings::lookup_hir_to_node): Change return
	type to an optional.
	* util/rust-hir-map.h: Adapt function prototype with the new return
	type.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>