Improve XHPAST syntax highlighter for "nc" and "nf" classes

Summary: For the crossrefernce stuff, tune the highlighter so it gets these
right more consistently.

Test Plan: Looked at some diffs with special rules for .nc and .nf, and they
seemed to generally apply to sensible symbols.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, nh, epriestley, jungejason

Differential Revision: 903