Add a method, Spawn#isOutputMandatory, to indicate whether a Spawn must create an output by the end of its execution. If not, a Spawn (like a test) may succeed without necessarily creating that output.

The 4 categories of actions that do this are:

1. Tests (tests can create XML and other files, but may not).
2. Java compilations with reduced classpaths (the initial compilation with a reduced classpath may fail, but produce a usable jdeps proto file, so the Spawn will claim that it succeeded so that the action can process the proto file).
3. Extra actions with a dummy output that is produced locally, not by the Spawn. However, by changing the outputs of the Spawn to be empty, this situation is avoided.
4. C++ compilations with coverage (a .gcno coverage file is not produced for an empty translation unit).

In particular, all SpawnActions' Spawns have #isMandatoryOutput always return true, and there should be no new cases of #isMandatoryOutput returning false besides the ones added in this CL.

PiperOrigin-RevId: 425616085