[implicit-instantiation] Always run Pierce's algorithm for implicit instantiation under LTI mode
[implicit-instantiation] Always run Pierce's algorithm for implicit instantiation under LTI mode

Summary:
Not running Pierce's algorithm is a temporary measure back when the implicit instantiation kit doesn't have access to hints. Now that we have hints and can evaluate them, we should always run Pierce.

Similar to contextual typing for arrays, we need to ignore errors when flowing return hint to avoid spurious double errors.

Changelog: [internal]

Reviewed By: mvitousek

Differential Revision: D40126913

fbshipit-source-id: b33c200fd1e8239f31a6894cab3c42c7eb205da9

mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow