Change lookup_hir_extern_item return type

Change lookup_hir_extern_item return type

Wrap the return type with an optional and make the return type a pair
with the parent hid.

gcc/rust/ChangeLog:

	* backend/rust-compile-resolve-path.cc (HIRCompileBase::query_compile):
	Adapt code around new return type.
	* checks/errors/rust-const-checker.cc (ConstChecker::check_function_call):
	Likewise.
	* checks/errors/rust-unsafe-checker.cc (UnsafeChecker::check_use_of_static):
	Likewise.
	(UnsafeChecker::check_function_call): Likewise.
	* typecheck/rust-type-util.cc (query_type): Likewise.
	* util/rust-hir-map.cc (Mappings::insert_hir_extern_item): Likewise.
	(Mappings::lookup_hir_extern_item): Change return type.
	* util/rust-hir-map.h: Update the function's prototype.

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