Re-enable MSVC C4091 diagnostic; NFC (#182500)
From MSDN:
https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4091?view=msvc-170
> 'keyword': ignored on left of 'type' when no variable is declared
This diagnostic was disabled in 0cba642a05ad0675f313e55fce29ce5273340a59
because the diagnostic was being hit in system headers under MSVC 2015.
That no longer appears to be the case from my local testing; this
diagnostic does not appear to be emitted.
