InternPool: remove memoized_decl

This is neither a type nor a value. Simplifies `addStrLit` as well as
the many places that switch on `InternPool.Key`.

This is a partial revert of bec29b9e498e08202679aa29a45dab2a06a69a1e.

src/codegen.zig

@@ -610,9 +610,7 @@ pub fn generateSymbol(
                 }
             }
         },
-        .memoized_decl,
-        .memoized_call,
-        => unreachable,
+        .memoized_call => unreachable,
     }
     return .ok;
 }