gccrs: Change return type of lookup_hir_extern_block

gccrs: Change return type of lookup_hir_extern_block

Change the return type to an optional in order to easily differentiate
between a null pointer and an missing value.

gcc/rust/ChangeLog:

	* checks/errors/rust-unsafe-checker.cc (UnsafeChecker::check_function_call):
	Adapt function call to new return type.
	* typecheck/rust-type-util.cc (query_type): Likewise.
	* util/rust-hir-map.cc (Mappings::insert_hir_extern_block): Likewise.
	(Mappings::lookup_hir_extern_block): Change return type to an optional.
	* util/rust-hir-map.h: Update the function's prototype.

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