[clang] Remove `HelpHidden` from the `-resource-dir` option (#163131)

Currently the "getter" option `-print-resource-dir` is visible when
typing `--help`, but the corresponding "setter" option `-resource-dir`
is not.

This option is useful when one is using clang on a non-standard
location, or when one is building a libtooling-based tool (e.g. IWYU)
based on a local clang build. In that case, we need to specify the
correct path to the resource directory for things to work.

Existing documentation already makes use of this option, for example
here:


https://clang.llvm.org/docs/StandardCPlusPlusModules.html#possible-issues-failed-to-find-system-headers

There is thus no reason to keep this option hidden from the help and
documentation.

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Co-authored-by: Carlos Gálvez <carlos.galvez@zenseact.com>

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