Distinguish the two kinds of token range.

When collecting tokens there are two kinds of range:
- a range relative to the parser's full token stream (which we get when
  we are parsing);
- a range relative to a single AST node's token stream (which we use
  within `LazyAttrTokenStreamImpl` when replacing tokens).

These are currently both represented with `Range<u32>` and it's easy to
mix them up -- until now I hadn't properly understood the difference.

This commit introduces `ParserRange` and `NodeRange` to distinguish
them. This also requires splitting `ReplaceRange` in two, giving the new
types `ParserReplacement` and `NodeReplacement`. (These latter two names
reduce the overloading of the word "range".)

The commit also rewrites some comments to be clearer.

The end result is a little more verbose, but much clearer.

compiler/rustc_builtin_macros/src/cfg_eval.rs

@@ -202,7 +202,7 @@ impl CfgEval<'_> {
         }
 
         // Now that we have our re-parsed `AttrTokenStream`, recursively configuring
-        // our attribute target will correctly the tokens as well.
+        // our attribute target will correctly configure the tokens as well.
         flat_map_annotatable(self, annotatable)
     }
 }