Check then callability using tryCallableResult as it forwards to thenTry anyway
Check then callability using tryCallableResult as it forwards to thenTry anyway

Summary: Switch to the correct callability check to match the internals. Without this change then is ambiguous and can still generate errors on polymorphic lambdas.

Reviewed By: yfeldblum

Differential Revision: D15147830

fbshipit-source-id: 3f577641495a24076e9ffa19deaad190452c420c

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly