typechecker: remove "copy" function hardcoding

Summary: This was the remnants of an old experiment for detecting
 copy-on-write occurrences statically that didn't pan out and was
 broken by the introduction of namespacing support.

Reviewed By: @int3

Differential Revision: D1624013
typechecker: more constants for hardcoded special names

Summary: Followup to {D1616902}:
 - user attributes
 - a few straggler callsites

Reviewed By: @int3

Differential Revision: D1619404