Add TransitionType enum and add checks that transitions aren't used

[katre]

inappropriately.

Fixes #13751.

[gregestren]

How do Starlark transitions fit into this? If you define a 1-1 Starlark transition and both apply it to a rule and an attribute, how does that still work? Is that because those don't show up as TransitionFactory instances in the StarlarkRuleClassFunctions object?

[gregestren]

Oh, those are handled at

bazel/src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkRuleClassFunctions.java

Line 409 in 525227e

if (cfg instanceof StarlarkDefinedConfigTransition) {

?

[katre]

Right, plain transitions don't run into this workflow. There's no way to make an actual TransitionFactory instance from Starlark, so this only applies to native code.

Native factories that don't care about the data type (like the transition factory for HostTransition) can use TransitionFactory.TransitionType.ANY to indicate that.