Expose ProvidePositionalOption for use in mlir.

[pschuh]

Summary

The motivation is to reuse the key value parsing logic here to parse instance specific pass options within the context of MLIR. The primary functionality exposed is the "," splitting for arrays and the logic for properly handling duplicate definitions of a single flag. See: https://groups.google.com/a/tensorflow.org/forum/#!topic/mlir/uROv8TX52Ts for more context.

[mehdi_amini]

LGTM

llvm/trunk/include/llvm/Support/CommandLine.h

 void ResetAllOptionOccurrences();
 
 /// Reset the command line parser back to its initial state.  This
 /// removes
 /// all options, categories, and subcommands and returns the parser to a state
 /// where no options are supported.
 void ResetCommandLineParser();
 
+/// Parses `Arg` into the option handler `Handler`.
+bool ProvidePositionalOption(Option *Handler, StringRef Arg, int i);
+
 } // end namespace cl
 
 } // end namespace llvm
 
 #endif // LLVM_SUPPORT_COMMANDLINE_H

llvm/trunk/lib/Support/CommandLine.cpp

while (NumAdditionalVals > 0) {
     if (CommaSeparateAndAddOccurrence(Handler, i, ArgName, Value, MultiArg))
       return true;
     MultiArg = true;
     --NumAdditionalVals;
   }
   return false;
 }
 
-static bool ProvidePositionalOption(Option *Handler, StringRef Arg, int i) {
+bool llvm::cl::ProvidePositionalOption(Option *Handler, StringRef Arg, int i) {
   int Dummy = i;
   return ProvideOption(Handler, Handler->ArgStr, Arg, 0, nullptr, Dummy);
 }
 
 // getOptionPred - Check to see if there are any options that satisfy the
 // specified predicate with names that are the prefixes in Name.  This is
 // checked by progressively stripping characters off of the name, checking to
 // see if there options that satisfy the predicate.  If we find one, return it,