[clang] [clang-tools-extra] [lld] [llvm] [llvm] Add subcommand support for OptTable (PR #155026)

[Petr Hosek via llvm-commits]

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petrhosek wrote:

Rather than having a directory in `examples` dedicated to just subcommands, I think we should a directory for `OptTable` and subcommands should be just one of the examples in there.

https://github.com/llvm/llvm-project/pull/155026