Add support for yield to XHPAST

Summary:
We are treating yield as an expression. This is rather unfortunate, but
unfortunately the entire implementation of yield is kind of hacky and there's no
particularly elegant way to do this.

Test Plan:
Linted something with yield, no syntax error

Reviewed By: epriestley
Reviewers: epriestley
CC: epriestley
Differential Revision: 98