gccrs: Change return type of lookup_impl_block_type

gccrs: Change return type of lookup_impl_block_type

Change the return type to an optional.

gcc/rust/ChangeLog:

	* typecheck/rust-type-util.cc (query_type): Adapt code to accomodate
	the new return type.
	* util/rust-hir-map.cc (Mappings::lookup_impl_block_type): Change
	the function's return type and remove the out pointer argument.
	* util/rust-hir-map.h: Update the function's prototype.

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