Unfortunate hhi partial revert

Summary: Partial revert of https://reviews.facebook.net/D20667 since it
causes too many errors in www, for `trim` and `date` specifically. Also
corrects the spelling of the filename and removes some `mixed`
annotations (`mixed` in an hhi is basically never what you want.)

See https://reviews.facebook.net/D20667 for discussion on how we can
deal with this better in an open-source world.

Reviewed By: @JoelMarcey

Differential Revision: D1461459
More properly support php 5.6 constant expressions.

Summary: Right now we allow arbitrary expressions for class constants and a restricted subset of expressions for global constants. (An incorrectly restricted one: it considers -foo() to be a constant.) This implements a more reasonable check and uses it for both global and class constants.

Reviewed By: @elgenie

Differential Revision: D1457074