[ADT] Remove deprecated variadic `StringSwitch::Cases` and `CasesLower` (#185191)
These overloads have been deprecated since October 2025.
Use the `std::initializer_list` overload instead: `.Cases({"a", "b"},
Value)`.
For more context, see https://github.com/llvm/llvm-project/pull/163117.
Assisted-by: claude
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>