Initial support for "$obj->:xhp-attr" syntax

Summary: This diff adds initial support for "$obj->:xhp-attr" syntax. As a first
step, this syntax will desugar to "$obj->getAttribute('xhp-attr')" to
support using the new "$obj->:xhp-attr" syntax to read attributes from XHP
objects.

Future plans include supporting "$obj->:xhp-attr" syntax for writing to
attributes on XHP objects and incorporating XHP attributes more with the
core runtime (ex. storing attributes inline inside the object similar to
normal object properties), but this will come later.

Reviewed By: @elgenie

Differential Revision: D1641268