Change lookup_hir_expr return type to optional

Change lookup_hir_expr return type to optional

Wrap the function's return type with an optional in order to
differentiate missing values from null pointers.

gcc/rust/ChangeLog:

	* backend/rust-mangle-v0.cc (v0_path): Adapt call site to new returned
	type.
	* util/rust-hir-map.cc (Mappings::lookup_hir_expr): Change the
	function's return type.
	* util/rust-hir-map.h: Update the function's prototype.

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