Implement Build Plan behavior "Affects Buildable"

Summary: Ref T13258. Make the "Affects Buildable" option actually work.

Test Plan:

• As in previous change, created a "wait for HTTP request" build plan and had it always run against every revision.
• Created revisions, waited a bit, then sent the build a "Fail" message, with different values of "Affects Buildable":
• "Always": Same behavior as today. Buildable waited for the build, then failed when it failed.
• "While Building": Buildable waited for the build, but passed even though it failed (buildable has green checkmark even though build is red):

{F6250359}

• "Never": Buildable passed immediately (buildable has green checkmark even though build is still running):

{F6250360}

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13258

Differential Revision: https://secure.phabricator.com/D20233