[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281290
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