[clang-tools-extra] [clang-doc][nfc] Clean up includes and namespace use in Representation sources (PR #208865)

[Paul Kirth via cfe-commits]

ilovepi wrote:

Nevermind. I split this out of a bigger stack, but the dead headers are only dead *after* the earlier patches.

https://github.com/llvm/llvm-project/pull/208865