Change lookup_hir_param return type with optional

Change lookup_hir_param return type with optional

Wrap the function's return type within an optional to differentiate
between a null ppointer and a missing value.

gcc/rust/ChangeLog:

	* util/rust-hir-map.cc (Mappings::insert_hir_param): Change call site
	to accomodate new return type.
	(Mappings::lookup_hir_param): Change the function's return type.
	* util/rust-hir-map.h: Updat ethe function's prototype.

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