[clang] [clang] introduce constexpr step limit opt-out (PR #160440)

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Tsche wrote:

Thanks. I've added the `(0 = no limit)` note to the driver's help text. The changes to UserManual.rst had already been made.

> We should say somewhere that `=0` will consume all available resources and somewhat discourage it.

Is this not implied? What else is it supposed to do if I disable self-imposed limits?

https://github.com/llvm/llvm-project/pull/160440