when emitting the epilog 'return 0' for top level code, mark it as autogenerated,

fixing <rdar://problem/19664185> Pointless "always evaluates to true" warning in while true loops at the top level

Also, merge test/SILPasses/diagnose_unreachable_two_while_loops.swift into SILPasses/unreachable_code.swift.


Swift SVN r27696

lib/SILGen/SILGen.cpp

@@ -812,9 +812,10 @@ public:
     if (sgm.TopLevelSGF) {
       // Write out the epilog.
       auto moduleLoc = RegularLocation::getModuleLocation();
-      auto returnInfo
-        = sgm.TopLevelSGF->emitEpilogBB(moduleLoc);
+      moduleLoc.markAutoGenerated();
+      auto returnInfo = sgm.TopLevelSGF->emitEpilogBB(moduleLoc);
       auto returnLoc = returnInfo.second;
+      returnLoc.markAutoGenerated();
 
       // Signal a normal return to the OS by returning 0.
       auto &B = sgm.TopLevelSGF->B;