Guard against unwinding in cleanup code

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -62,6 +62,9 @@ pub struct FunctionCx<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> {
     /// Cached unreachable block
     unreachable_block: Option<Bx::BasicBlock>,
 
+    /// Cached double unwind guarding block
+    double_unwind_guard: Option<Bx::BasicBlock>,
+
     /// The location where each MIR arg/var/tmp/ret is stored. This is
     /// usually an `PlaceRef` representing an alloca, but not always:
     /// sometimes we can skip the alloca and just store the value
@@ -169,6 +172,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         personality_slot: None,
         cached_llbbs,
         unreachable_block: None,
+        double_unwind_guard: None,
         cleanup_kinds,
         landing_pads: IndexVec::from_elem(None, mir.basic_blocks()),
         funclets: IndexVec::from_fn_n(|_| None, mir.basic_blocks().len()),