gccrs: Change lookup_hir_struct_field return type

gccrs: Change lookup_hir_struct_field return type

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

gcc/rust/ChangeLog:

	* util/rust-hir-map.cc (Mappings::insert_hir_struct_field): Change
	call site to accomodate new return type.
	(Mappings::lookup_hir_struct_field): 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>