Support: Fix option handling when using cl::Required with aliasopt

Until now, attempting to create an alias of a required option would
complain if the user supplied the alias, because the required option
didn't have a value. Similarly, if you said the alias was required,
then using the base option would complain that the alias wasn't
supplied. Lastly, if you put required on both, *neither* option would
work.

By changning alias to overload addOccurrence and setting cl::Required
on the original option, we can get this to behave in a more useful
way. I've also added a test and updated a user that was getting this
wrong.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212986 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-profdata/llvm-profdata.cpp

@@ -38,9 +38,9 @@ int merge_main(int argc, const char *argv[]) {
                                cl::desc("<filenames...>"));
 
   cl::opt<std::string> OutputFilename("output", cl::value_desc("output"),
-                                      cl::init("-"),
+                                      cl::init("-"), cl::Required,
                                       cl::desc("Output file"));
-  cl::alias OutputFilenameA("o", cl::desc("Alias for --output"), cl::Required,
+  cl::alias OutputFilenameA("o", cl::desc("Alias for --output"),
                             cl::aliasopt(OutputFilename));
 
   cl::ParseCommandLineOptions(argc, argv, "LLVM profile data merger\n");