abstract constants: preclass/class emission [2/2]

abstract constants: preclass/class emission [2/2]

Summary: `abstract const` in a class has the same general purpose as an
 abstract method: prevent a class from being 'concrete' unless it
 defines that constant. Trying to reference a value of an abstract
 constant remains a fatal (as if it were not defined at all).

 PHP5 doesn't allow interface constants to be redefined in any
 implementing class. In contrast, this diff allows interface/abstract
 class constants to be defined as `abstract` in a addition to regular
 definitions. This allows (and for non-abstract children, forces)
 extending classes to provide concrete definitions for those
 constants. This diff does not touch the prohibition on constants in
 traits.

 Between preclass emitter, preclass, class, and hhbbc, we have an
 impressive number of structs that represent constants. In all of
 these, the ## val ## of a const is changed from being a TypedValue to
 a ##folly::optional<TypedValue>##, with ##folly::none## representing
 an abstract constant definition. For constant values,
 ##KindOfUninit## is already taken as the representation values
 requiring runtime calculations (e.g. 'const X = self::Y;').

 Note: all this diff does for runtime reflection support is actively
 hide the abstract constant definitions.

Reviewed By: @jdelong

Differential Revision: D1693734

Signature: t1:1693734:1416989210:8f15fa8069018553973f0b285315f02330096323