ASTMangler: Verify that debug manglings round-trip

Add an IRGen flag to disable this verification, since it doesn't work from within
lldb itself for some reason, and I don't want to investigate it right now.

lib/IRGen/IRGenDebugInfo.cpp

@@ -698,7 +698,8 @@ private:
 
     Mangle::ASTMangler Mangler;
     std::string Name = Mangler.mangleTypeForDebugger(
-        Ty, DbgTy.getDeclContext());
+        Ty, DbgTy.getDeclContext(),
+        !Opts.DisableRoundTripDebugTypes);
     return BumpAllocatedString(Name);
   }