<xthreads.h>: Use enum class _Thrd_result for type safety

[StephanTLavavej]

I was briefly investigating #292 again due to a related user report, where it turned out that that specific problem was fixed by #3721. For the general problem of defending the STL against all unconstrained operator overloads, it's unclear whether we can practically solve it, or whether we should devote significant effort towards doing so (considering that it isn't a regression, and that plenty of UCRT/VCRuntime/PPLTasks headers are also affected). I note this because I don't want to encourage a bunch of follow-up PRs in this area.

However, I noticed that <xthreads.h> uses an unnamed, unscoped enumeration for its result codes, which are returned as int. This came to my attention because it leads to ints being compared to these enumerators (hence this can be hijacked by unconstrained operator overloads), but I decided to do something about it because it's a bad practice even aside from the operator overloading issue.

The most important observation is that all of these types and functions (except _Throw_C_error() in thread0.cpp, discussed below) are extern "C", so we can upgrade this unscoped enum to a scoped enum without breaking ABI or affecting the DLL's export surface, as long as we keep the underlying type the same (just int) and don't change the enumerator values (obvious). In addition to this semi-mechanical transformation, I'm shortening the comment // TRANSITION, ABI: Always returns _Thrd_success (with occasional case variation) to // TRANSITION, ABI: Always succeeds in order to avoid wrapping.

Finally, because it throws an exception, the preserved-for-bincompat _Throw_C_error() in thread0.cpp is C++ mangled, not extern "C". Therefore, it must continue to take int code, and must static_cast<_Thrd_result>(code) within.

I've verified with dumpbin /exports that the export surfaces of the release and debug DLLs are unchanged.

[StephanTLavavej]

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