Re-enable MSVC C4709 diagnostic; NFC (#182514)
From MSDN:
https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-4-c4709?view=msvc-170
> comma operator within array index expression
This is a useful diagnostic to have enabled in general. It was disabled
in 02e1f1915f94413a64efd295fa3dc63d40046fcf because of false positives,
but those appear to have been resolved. Enabling the diagnostic locally
does not emit any diagnostics.
