Add a clarifying comment in Flow Analysis' VisitSwitchBlock.

src/Compilers/CSharp/Portable/FlowAnalysis/PreciseAbstractFlowPass.cs

@@ -1669,6 +1669,8 @@ private void VisitSwitchBlock(BoundSwitchStatement node)
             for (var iSection = 0; iSection <= iLastSection; iSection++)
             {
                 VisitSwitchSection(switchSections[iSection], iSection == iLastSection);
+                // Even though it is illegal for the end of a switch section to be reachable, in erroneous
+                // code it may be reachable.  We treat that as an implicit break (branch to afterSwitchState).
                 IntersectWith(ref afterSwitchState, ref this.State);
             }