[libSyntax] Make RawSyntax a struct

SourcePresence and ID are already shared between the two node kinds and
the node's length will soon be cached in RawSyntax as well. By making it
a struct, we will be able to compute the node's length when it is being
constructed in the initialiser.

tools/SwiftSyntax/SwiftSyntax.swift

@@ -78,10 +78,7 @@ public final class SyntaxTreeDeserializer {
   }
 
   private func addToLookupTable(_ node: RawSyntax) {
-    guard let id = node.id else {
-      return
-    }
-    nodeLookupTable[id] = node
+    nodeLookupTable[node.id] = node
   }
 }