Expand use of IndexBase (#2436)

Initially I'd added this to lexer, this includes parser and semantics. Also adds ComparableIndexBase to unify a few common cases where <> comparisons are supported.

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/semantics/semantics_node.cpp

@@ -31,7 +31,7 @@ void SemanticsNode::Print(llvm::raw_ostream& out) const {
 #include "toolchain/semantics/semantics_node_kind.def"
   }
   out << ")";
-  if (type_.id != -1) {
+  if (type_.index != -1) {
     out << ": " << type_;
   }
 }