[clang] [clang-format]: Add `Custom` to `ShortFunctionStyle`; add new AllowShortFunctionsOnASingleLineOptions for granular setup (PR #134337)

[Owen Pan via cfe-commits]

https://github.com/owenca commented:

If there are any new test cases including those for only `Other == true`, can you move them to a future patch?

https://github.com/llvm/llvm-project/pull/134337