Make XHPASTSyntaxHighlighter more robust

Summary:

• Also store symbol name as a DOM attribute, to allow cases where the symbol name isn't simply the text content of the <span>.
• Check for interesting symbols first, to allow cases where the symbol doesn't have type T_STRING.

These changes support new features internal to Facebook, though they
also seem worthwhile in their own right.

Test Plan: Browse Diffusion. Examine DOM.

Reviewers: epriestley

Reviewed By: epriestley

CC: nh, aran, Korvin

Maniphest Tasks: T1602

Differential Revision: https://secure.phabricator.com/D3226