[Exclusivity] Reenable AccessEnforcementSelection assert.

Author: atrick

lib/SILOptimizer/Mandatory/AccessEnforcementSelection.cpp

@@ -382,12 +382,7 @@ struct AccessEnforcementSelection : SILFunctionTransform {
     // dependent BeginAccess).
     //
     // Running before diagnostic constant propagation requires handling 'undef'.
-    //
-    // FIXME: This assert will eventually be important, but for now it's safe to
-    // disable until this problem is resolved:
-    //   <rdar://problem/31797132> [Exclusivity] lldb asserts in
-    //   AccessEnforcementSelector
-    // assert(isa<AllocStackInst>(address) || isa<SILUndef>(address));
+    assert(isa<AllocStackInst>(address) || isa<SILUndef>(address));
     setStaticEnforcement(access);
   }