[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.cpp

@@ -85,7 +85,7 @@ cl::opt<bool> Examine("examine",
   cl::cat(ToolTemplateCategory));
 
 cl::opt<bool> DashDash("-",
-  cl::desc("Separator between hipify-clang and clang options;\ndon't specify if there are no clang options."),
+  cl::desc("Separator between hipify-clang and clang options;\ndon't specify if there are no clang options"),
   cl::value_desc("--"),
   cl::cat(ToolTemplateCategory));
 
@@ -96,4 +96,11 @@ cl::list<std::string> I("I",
   cl::Prefix,
   cl::cat(ToolTemplateCategory));
 
+cl::list<std::string> D("D",
+  cl::desc("Define <macro> to <value> or 1 if <value> omitted"),
+  cl::value_desc("macro>=<value"),
+  cl::ZeroOrMore,
+  cl::Prefix,
+  cl::cat(ToolTemplateCategory));
+
 cl::extrahelp CommonHelp(ct::CommonOptionsParser::HelpMessage);