APICompat is skipped on incremental build after failures

[ericstj]

Since APICompat is triggered only when CoreCompile runs it won't rerun even if it fails.

To fix this we can emit a file when APIComat fails and have that be appended to the set of inputs to corecompile. That way when APICompat fails we'll invalidate CoreCompile's up-to-date check. On success we can delete that file.

[ericstj]

ApiCompat tries to handle this by touching an input file but it doesn't seem to work consistently.

[ericstj]

It's trying to force this by touching AssemblyInfoFile but AssemblyInfoFile may not always exist. Instead it should have it's own file that it adds to custom file inputs.

[ericstj]

#2672 added a semaphore file that fixed this.