[clang] [clang-format] Add -r option for recursing into directories (PR #160299)

[James Henderson via cfe-commits]

jh7370 wrote:

The request for this has been open for two and a half years. I've needed it several times myself and I've not seen a convincing argument that we shouldn't support this (writing a script that recurses a directory tree to run clang-format on each level is not a one line command invocation). There is prior art in the form of things like the go formatter, plus plenty of other tools recurse through directory trees, either when explicitly asked or by default, so I think we should do it.

https://github.com/llvm/llvm-project/pull/160299