Fix -Wno-error= parsing in clang-format.

As noted in https://reviews.llvm.org/D86137#2460135 parsing of
the clang-format parameter -Wno-error=unknown fails.
This currently is done by having `-Wno-error=unknown` as an option.
In this patch this is changed to make `-Wno-error=` parse an enum into a bit set.
This way the parsing is fixed and also we can possibly add new options easily.

Reviewed By: MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D93459