[clang] [llvm] [mlir] [Support] Remove cl::DefaultOption (PR #225609)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> At a glance (I haven't tested to confirm), it looks like some tools a) have lost the "hidden" nature of -h and b) have lost the categorisation of the option. My instinct says that the latter is fine as the categorisation was probably a mistake in the first place. The loss of the hidden property is more of a concern if a tool usually hides short aliases in the help text. Thoughts?

`-h` is still hidden. `--help`/`--help-list` may now be listed under the `Generic Options:` category. 
I don't think this detail matters, and these tools should eventually migrate to OptTable.

https://github.com/llvm/llvm-project/pull/225609