Fixes: 56c4971d33230607a7329bb831b6c8588231e872
If the default target triple uses visualstudio::Linker::ConstructJob,
when a MSVC installation cannot be found, there will be a
-Wmsvc-not-found diagnostic, which is turned to an error due to -Werror.
We have many driver tests that don't specify --target= and would get a
-Wmsvc-not-found warning, but this might be the only that uses -Werror
and is not skipped by a `UNSUPPORTED`.
After https://reviews.llvm.org/D156363 changed -### to exist with code 1
if hasErrorOccurred, we can use -Werror to assert no warnings. This is
more reliable than a NOT pattern, which may go stale (when the
diagnostic is updated) without being noticed.
