Clear inner_attr_ranges regularly.

There's a comment saying we don't do it for performance reasons, but it
doesn't actually affect performance.

The commit also tweaks the control flow, to make clearer that two code
paths are mutually exclusive.

compiler/rustc_parse/src/parser/attr_wrapper.rs

@@ -352,15 +352,10 @@ impl<'a> Parser<'a> {
             let target = AttrsTarget { attrs: final_attrs.iter().cloned().collect(), tokens };
             self.capture_state.replace_ranges.push((start_pos..end_pos, Some(target)));
             self.capture_state.replace_ranges.extend(inner_attr_replace_ranges);
-        }
-
-        // Only clear our `replace_ranges` when we're finished capturing entirely.
-        if matches!(self.capture_state.capturing, Capturing::No) {
+        } else if matches!(self.capture_state.capturing, Capturing::No) {
+            // Only clear the ranges once we've finished capturing entirely.
             self.capture_state.replace_ranges.clear();
-            // We don't clear `inner_attr_ranges`, as doing so repeatedly
-            // had a measurable performance impact. Most inner attributes that
-            // we insert will get removed - when we drop the parser, we'll free
-            // up the memory used by any attributes that we didn't remove from the map.
+            self.capture_state.inner_attr_ranges.clear();
         }
         Ok(ret)
     }