Identifier usage analyzer should count disappeared uses

[jackfirth]

The implementation of identifier-usage-analyzer currently doesn't see disappeared uses, as recorded by macros in the 'disappeared-use syntax property. This test should pass:

#lang resyntax/test
header: - #lang racket/base

analysis-test: "struct-copy disappeared use counted"
----------------------------------------
(struct point (x y z))
(define (f pt)
  (struct-copy point pt [y 100]))
----------------------------------------
@within - (struct point (x y z))
@inspect - point
@property usage-count
@assert 1