[ADT] Mark StringSwitch Cases with 6+ arguments as deprecated. NFC. (#163405)

Switch to the `.Cases({S0, S1, ...}, Value)` overload instead.

Update existing uses affected by the deprecation and the surrounding
code (for consistency).

This is a part of a larger cleanup of StringSwitch. The goal is to
eventually deprecate all manually-enumerated overloads of Cases with a
hardcoded number of case values (in favor of passing them via an
initializer list). You can find the full explanation here:
https://github.com/llvm/llvm-project/pull/163117.

Start small (6+ arguments) to keep the number of changes manageable.

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