[Support][CommandLine] Add cl::getRegisteredSubcommands()

This should allow users of the library to get a range to iterate through
all the subcommands that are registered to the global parser. This
allows users to define subcommands in libraries that self-register to
have dispatch done at a different stage (like main). It allows for
writing code like the following:

    for (auto *S : cl::getRegisteredSubcommands()) {
      if (*S) {
	// Dispatch on S->getName().
      }
    }

This change also contains tests that show this usage pattern.

Reviewers: zturner, dblaikie, echristo

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D24489

llvm-svn: 283296