Feature request

While debugging #5825 , I'm discovering a range of interesting problems, mostly due to Windows' slow I/O.

I found it interesting that StubValidator doesn't use the result cache at all, because validating stubs takes nearly 25% of the pre-analysis step time (under xdebug that's about 1 second out of 4; at normal runtime it would be closer to 300ms).

This might seem insignificant, but I find it odd that stubs aren't remembered by the result cache like normal files.

I'm looking to save all the pre-analyse time I can find here, because this pre-analyse lag is a pretty huge annoyance for using PHPStan on Windows (particularly when integrated with PHPStorm, because it takes quite some time to respond - 300ms is a pretty significant amount of time in that case).