typechecker: UNSAFE_Construct

Summary: ... to allow explicit escape of `<<ConsistentConstruct>>`. In
 the absence of a "## // decl ## *this* declaration" syntax, this is
 better than the alternative of making entire class hierarchies
 type-unsound.

Reviewed By: @jwatzman

Differential Revision: D1481454
typechecker: typing new static() [3/3]

Summary: `new static` is now allowed only on where the class hierarchy
 has required consistently compatible constructor signatures due to
 either properties of the class/`__construct` method, or due to the
 use of the `<<ConsistentConstruct>>` attribute.

Reviewed By: @jwatzman

Differential Revision: D1466669