[ADT] Deprecate StringSwitch Cases with 3+ args. NFC. (#165119)

Suggest the `initializer_list` overload instead.

3+ args is an arbitrary number that allows for incremental depreciation
without having to update too many call sites.

For more context, see https://github.com/llvm/llvm-project/pull/163117.

GitOrigin-RevId: 3526bb099e79b1217bb8afbf9403b11528c384a6