[clang-tools-extra] Restore clang-doc arena allocation (PR #197595)

[Paul Kirth via cfe-commits]

ilovepi wrote:

@evelez7 I haven't tried this on Darwin yet, and won't be able to until tomorrow when I'm back in the office. It did pass on Linux under ASan + UBSan.

https://github.com/llvm/llvm-project/pull/197595