Wait on condition variables independently on system time

[AlexGuteniev]

Fix #718

Possibly create chaos (I hope not).

I made new core header, since making <xthreads.h> non-core looked hard.

Thanks @achabense for the first step in this direction.

[StephanTLavavej]

⚠️ Note to self:

This will need the usual MSVC-internal changes to add a new header.

[AlexGuteniev]

One of scary things here is locking down the _Cnd_t and _Mtx_t.
We cannot further change them after this change, as the assumption on their structure are embedded into import library.

[AlexGuteniev]

One of scary things here is locking down the _Cnd_t and _Mtx_t. We cannot further change them after this change, as the assumption on their structure are embedded into import library.

The alternative could be a satellite DLL, this makes the change possible without core header extraction, but this can be more cumbersome overall.

[StephanTLavavej]

I think I'm fine with that limitation - the headers (not just the import lib) already know about the mutex representation, and the condition variable representation has been boiled down to its essentials now.

Satellite DLLs are really annoying. I don't think we'll ever use them again unless we're absolutely forced to.

There's one more transformation I want to perform (I want to get rid of the forward declaration squirrelliness) but that won't affect representation.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for fixing this notorious bug that we previously thought was unfixable! ⭐ 😻 🎉