Ignore a test on MSVC

The MSVC SEH is still not implemented, so we go ahead and ignore it.

Author: nagisa

src/test/run-fail/mir_drop_panics.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 #![feature(rustc_attrs)]
+
+// ignore-msvc: FIXME(#30941)
 // error-pattern:panic 1
 // error-pattern:drop 2
 use std::io::{self, Write};