options for a custom subcommand.

Context:

  In https://lists.llvm.org/pipermail/llvm-dev/2016-June/101804.html and commit 07670b3, cl::SubCommand is introduced.

  Options that don't specify subcommand goes intoa special 'top level' subcommand.

Motivating Use Case:
  The motivating use case is to refactor llvm-profdata to use cl::SubCommand to organize subcommands. See pr/71328. A valid use case that's not supported before this patch

  // show-option{1,2} are associated with 'show' subcommand.
  // top-level-option3 is in top-level subcomand (e.g., `profile-isfs` in SampleProfReader.cpp)
  llvm-profdata show --show-option1 --show-option2 --top-level-option3

  - Before this patch, option handler look-up will fail with the following error message
    "Unknown command line argument --top-level-option3".
  - After this patch, option handler look-up will look up in sub-command options first,
    and use top-level subcommand as a fallback, so 'top-level-option3' is parsed correctly.

…ith upper case letter; and use std::pair initializer.