[benchmark] Reapply fix for -Wcovered-switch-default warning

This reapplies a fix from 948ce4e6,
whichn't originally submitted upstream. I has now been merged upstream
though, in https://github.com/google/benchmark/pull/1302.

When benchmarks were unified in
5dda2efd, it lost this change,
but it also lost another local modification, where benchmark's
CMakeLists.txt was modified to comment out adding -Werror.
(This change was part of the original import in
0addd170.)

As the benchmark library is built automatically by default, when
building all of LLVM (contrary to the copy in libcxx, which wasn't
built by default), building it with -Werror by default is very brittle.

This fixes building LLVM with MinGW. (It wasn't broken in MSVC
mode, as the benchmark library doesn't add -Werror or anything
equivalent in MSVC mode, and it's unclear if this warning is
enabled in that mode at all.)

Differential Revision: https://reviews.llvm.org/D115434