gccrs: Change return type of lookup_hir_trait_item

gccrs: Change return type of lookup_hir_trait_item

Change the return type to an optional instead of returning a null
pointer. This allows easier tracking of rogue null pointers in the
map. This commit also fixes a bug in trait associated function mangling,
the function was using an already invalidated pointer.

gcc/rust/ChangeLog:

	* backend/rust-compile-resolve-path.cc (HIRCompileBase::query_compile):
	Adapt return type to new optional.
	* backend/rust-mangle-v0.cc (v0_function_path): Change prototype to use
	the generic arguments vector instead of the whole HIR function.
	(v0_path): Fix a bug with a null pointer being used to create the
	trait function mangling.
	* util/rust-hir-map.cc (Mappings::insert_hir_trait_item): Adapt code
	to new return type.
	(Mappings::lookup_hir_trait_item): Change the return type of the
	function to an optional.
	* util/rust-hir-map.h: Update the function's prototype.

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