Change result_id to use is_valid instead of an optional. (#2449)

Incrementally shrinks size, but should also make the errors a bit clearer with better context.

toolchain/semantics/semantics_node.h

@@ -23,6 +23,7 @@ struct SemanticsNodeId : public IndexBase {
   static auto MakeCrossReference(int32_t index) -> SemanticsNodeId {
     return SemanticsNodeId(index | CrossReferenceBit);
   }
+
   // Constructs a cross-reference node ID for a builtin. This relies on
   // SemanticsIR guarantees for builtin cross-reference placement.
   static auto MakeBuiltinReference(SemanticsBuiltinKind kind)
@@ -30,6 +31,9 @@ struct SemanticsNodeId : public IndexBase {
     return MakeCrossReference(kind.AsInt());
   }
 
+  // Constructs an explicitly invalid instance.
+  static auto MakeInvalid() -> SemanticsNodeId { return SemanticsNodeId(); }
+
   using IndexBase::IndexBase;
 
   auto is_cross_reference() const -> bool { return index & CrossReferenceBit; }