R1:f97df9ebea90

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…