Fix skipping of flags in getClangStripDependencyFileAdjuster

[kastiglione]

Summary

The ArgumentsAdjuster returned from getClangStripDependencyFileAdjuster will
skip dependency flags, and also their associated values for those flags that
take an argument. This change corrects the handling of the -MD and -MMD
flags, which do not take an argument.

[kastiglione]

Add a continue;

[alexander-shaposhnikov]

probably a test would be useful

[kastiglione]

Add tests

lib/Tooling/ArgumentsAdjusters.cpp

return [](const CommandLineArguments &Args, StringRef /*unused*/) {
     CommandLineArguments AdjustedArgs;
     for (size_t i = 0, e = Args.size(); i < e; ++i) {
       StringRef Arg = Args[i];
       // All dependency-file options begin with -M. These include -MM,
       // -MF, -MG, -MP, -MT, -MQ, -MD, and -MMD.
       if (!Arg.startswith("-M"))
         AdjustedArgs.push_back(Args[i]);
 
-      if ((Arg == "-MF") || (Arg == "-MT") || (Arg == "-MQ") ||
-          (Arg == "-MD") || (Arg == "-MMD")) {
-        // Output is specified as -MX foo. Skip the next argument also.
+      if (Arg == "-MF" || Arg == "-MT" || Arg == "-MQ")
+        // These flags take an argument: -MX foo. Skip the next argument also.
         ++i;
-      }
-    }
     return AdjustedArgs;
   };
 }
 
 ArgumentsAdjuster getInsertArgumentAdjuster(const CommandLineArguments &Extra,
                                             ArgumentInsertPosition Pos) {
   return [Extra, Pos](const CommandLineArguments &Args, StringRef /*unused*/) {
     CommandLineArguments Return(Args);