[clang-tools-extra] [clang-doc] Move Info types into arenas (PR #190054)

[Paul Kirth via cfe-commits]

ilovepi wrote:

@dyung are the bots still broken after #191570? We had some uninitialzed reads that got fixed up in that pr. It tripped msan on the next pr in the stack, too but I think they are all caused by the same root issue addressed in that follow up PR. 

https://github.com/llvm/llvm-project/pull/190054