[Diagnostics] Don't drop a statically set NoWarningAsError flag during option processing

When a -W<diag> option is given on the command line, and the corresponding diagnostic has
the NoWarnOnError flag set, prevent the flag from being dropped when the severity is reevaluated.
This fixes PR51837.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D109981