[HIPIFY] Introduce '-D' option as hipify-clang's one

+ '-D' might be set as hipify-clang option (before separator '--' or without specifying separator at all);
+ '-D' as a clang option might be specified as well (after options separator '--');
+ All defines specified as hipify-clang defines (before options separator '--') will be set after clang's ones and will redefine the same clang's ones.

hipify-clang/src/ArgParse.h

@@ -34,6 +34,7 @@ extern cl::opt<std::string> OutputDir;
 extern cl::opt<std::string> TemporaryDir;
 extern cl::opt<std::string> CudaPath;
 extern cl::list<std::string> I;
+extern cl::list<std::string> D;
 extern cl::opt<bool> Inplace;
 extern cl::opt<bool> SaveTemps;
 extern cl::opt<bool> NoBackup;