pfff: do not generate a __var__ fake classname for xhp attribute with var

Summary:
Gary reported this false positive in cmf:
>>> Lint for flib/profile/browser/ui/selector/selector.php:

   Error  (checkModule) UseOfUndefined
    Undefined class: __var__. You may need to run 'arc build' for cmf  to
    catchup with newly introduced functions/classes/constants. If the entity
    is in a newly created thrift package, you may also need to run
    scripts/flib/ThriftAutoloadBuilder.php.  (read
    https://our.intern.facebook.com/intern/wiki/index.php/Pfff/CheckModuleFast
    and then email pad@fb.com if this is a false positive)

              11   attribute
              12     :ui:selector-DEPRECATED,
              13     ProfileBrowserSelectorData data @required,
    >>>       14     var listview @required;
              15
              16   category %profile-browser:selector;

This patch fixes that by avoiding introducing a fake hint type,
that then get visited and trigger a false positf

Test Plan: make test in pfff/facebook/check_module now works

Reviewers: pieter, zoel

Reviewed By: pieter

CC: gyngve

Differential Revision: https://phabricator.fb.com/D764862