[ADT] Deprecate StringSwitch Cases with 3+ args. NFC.

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 llvm#163117.

Author: kuhar

llvm/include/llvm/ADT/StringSwitch.h

@@ -93,6 +93,7 @@ class StringSwitch {
     return CasesImpl({S0, S1}, Value);
   }
 
+  [[deprecated("Pass cases in std::initializer_list instead")]]
   StringSwitch &Cases(StringLiteral S0, StringLiteral S1, StringLiteral S2,
                       T Value) {
     return CasesImpl({S0, S1, S2}, Value);
@@ -176,6 +177,7 @@ class StringSwitch {
     return CasesLowerImpl({S0, S1}, Value);
   }
 
+  [[deprecated("Pass cases in std::initializer_list instead")]]
   StringSwitch &CasesLower(StringLiteral S0, StringLiteral S1, StringLiteral S2,
                            T Value) {
     return CasesLowerImpl({S0, S1, S2}, Value);