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

[David Spickett via cfe-commits]

DavidSpickett wrote:

Failure is `Clang-Unit.Basic/_/BasicTests_exe/SuppressionMappingTest/LongestMatchWins` which is unrelated and I know I've seen fail on the bots. I think it has been reverted or fixed, so if you rebase CI should pass.

If not then go ahead and merge.

https://github.com/llvm/llvm-project/pull/163131