Re-enable MSVC C4592 diagnostic; NFC (#182503)
This diagnostic is no longer documented on MSDN, but the diagnostic text
was:
> ''var': symbol will be dynamically initialized (implementation
limitation)
This was disabled in 5cbf37fe3703ee7744f864a53df443ab97e29af7 to work
around false positives with MSVC 2015 Update 1. I believe those false
positives have been fixed and this diagnostic no longer needs to be
disabled. Local testing shows it is not emitted.
